STATE OF TECHNOLOGY 2017 FLOW MEASUREMENT

SPECIAL REPORT

STATE OF TECHNOLOGY 2017

FLOW MEASUREMENT

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State of Technology 2017: Flow Measurement 3

When and how to use pulse width modulation of controller outputs 5

Getting the most out of valve positioners 9

Maximum rangeability of linear valves 14

Orifice flowmeter rangeability 20

Flow of sandy condensate 25

Piercing thick flow problems 28

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TABLE OF CONTENTS

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When and how to use pulse width modulation of controller outputsGreg McMillan

In some applications, throttling of the manipulated flows is difficult or impossible. In the

biochemical industry, where precise (good resolution and sensitivity) throttling valves

without any crevices (to meet sanitary requirements) are rather limited (there are ex-

ceptions, such as the Fisher Baumann 83000-89000 series). Often, pulse width modulation

(PWM) is used to turn nutrient and reagent pumps on and off. In the chemical industry,

PWM is used to open and close valves whose trim would plug or whose stem would stick

if throttled. The sudden burst of flow from on-off action helps flush out the trim and wipe

the stem clean. PWM is correspondingly used for small reagent flows, corrosive fluids, and

slurries. It is also used to prevent flashing by a valve position that ensures a pressure drop

above the critical pressure drop. PWM is also used in temperature loops to turn heaters on

and off. Here, it is commonly called “time proportioning control” but the action is principally

the same. Temperature loops for extruders, silicon crystal pullers and environmental cham-

bers often use this technique.

All the applications of PWM have one thing in common; a capacity to filter or dilute the pulses

so that they do not appear as measurement noise in the controlled variable. PWM provides a

train of pulses that show up as a sawtooth in the measurement unless attenuated. The mass of

fluid and metal in a reactor, extruder, or crystal puller and mass of air in an environment cham-

ber must be large enough and the maximum pulse width small enough so that the amplitude of

the sawtooth seen is negligible.



The rangeability achieved by PWM is basically

equal to the maximum pulse width divided by

the minimum pulse width. Since a valve must

reach a set position and the pump must reach

a set speed during the pulse, the minimum

pulse width is fixed by the pre-stroke dead-

time and stroking time of the valve or the rate

limiting of the speed and acceleration time of

pump. Usually, four seconds is adequate for

small valves and pumps. The maximum pulse

width is the pulse cycle time when the pulse

is almost continuously on. Since the pulse

cycle time also sets the time between succes-

sive pulses, it adds a maximum deadtime to

the loop that is about equal to the cycle time

when the pulse is almost continuously off.

For an average controller output of 50%, the

deadtime added is about ½ the cycle time.

Thus, the cycle time chosen represents a

compromise of the desire to maximize range-

ability and minimize the sawtooth amplitude

seen in measurement and minimize loop

deadtime. An additional consideration is the

wear and tear on the final control element.

Pumps, agitator and motor driven valves

have a maximum duty cycle that must not be

exceeded. Also, heaters in the motor starter

will trip for too short a cycle time because the

temperature rise from lack of cool down is

equated to an overload current. For valves,

periodic opening and closing will eventually

cause packing, seat, seal or trim failure.

The consequences and methods of mitiga-

tion of pulses are discussed in the 12/15/2014

Control Talk Blog, “Controller Attenuation

and Resonance Tips.” A simple equation to

predict the amplitude of pulses after attenua-

tion by the process or a filter that are seen by

the controller is discussed in the 12/02/2104

Control Talk Blog, “Measurement Attenuation

and Deception Tips.”

The generation of a pulse train is done by

special output cards or by the configura-

tion of function blocks on the PID output.

The heart of the configuration is a ramp that

resets itself periodically. An integrator (INT)

function block is employed to generate the

ramp. The configuration depends upon what

version of Distributed Control System (DCS)

or Programmable Logic Controller (PLC)

is used. For an integrator that will ramp up

towards a setpoint, the input to the integra-

tor is set equal to 100% divided by the de-

sired cycle time. The integrator setpoint is set

slightly larger than 100%. The ramp “on” time

or pulse width is determined by comparing

percent ramp value (integrator output) to

the percent controller output via a high signal

monitor (HSM) block. When the ramp value

exceeds the controller output, a discrete is

see equal to one (true), which opens a dis-

crete output or transfers in an analog output

value that corresponds to the closed valve

position or minimum speed. If the control-

ler output drops below the minimum pulse

width, the pulse is turned off by transferring

in a negative value before the ramp value is

used as the operand of the HSM block on the

output of the INT block. The PID low output

limit should be set to be slightly less than this

http://www.controlglobal.com/blogs/controltalkblog/controller-attenuation-and-resonance-tips/

http://www.controlglobal.com/blogs/controltalkblog/controller-attenuation-and-resonance-tips/

http://www.controlglobal.com/blogs/controltalkblog/measurement-attenuation-and-deception-tips/

http://www.controlglobal.com/blogs/controltalkblog/measurement-attenuation-and-deception-tips/



minimum pulse width. The functionality of

blocks depends upon the DCS or PLC used

and any configuration must be extensively

tested before being used online.

For viscous fluids and slurries, a precise con-

trol valve may be continuously throttled until

the valve position gets so small that laminar

flow or plugging can occur. At this point (e.g.,

below 20% PID output), PWM starts. The

throttling valve position then stays open (e.g.,

20%) and an inexpensive on-off valve in se-

ries with the control valve is open and closed

by PWM.

There are many other applications of PWM.

Pulsed flows have been shown to increase

the yield of reactors, the separation in distil-

lation columns and the combustion efficiency

of burners. Pulse reagent flow has been very

successfully used to mimic a titration for

batch pH control. While many well designed

pulsed strategies can work for this applica-

tion, PWM on a proportional-only pH PID

controller retains a conventional operator

interface via the PID operator faceplate and

tuning via the PID gain setting. Also, the con-

troller output can be transferred in for the an-

alog output to reduce batch processing time

by providing pulses that are not only longer

but that are also larger (pulse width plus

pulse amplitude modulation). The gain of the

manipulated variable is now nonlinear and is

proportional to the controller output. Howev-

er, for proportional-only control of batch pH

processes, this gain change may be advanta-

geous and offset the low pH process gain

from the operating point being on the flat

portion of the titration curve at the beginning

of the batch cycle moving to the steep part

of the curve at the end of the batch. This is an

example of how a continuous control tech-

nique is also useful for batch processing.

PWM also dramatically reduces the effects of

deadband and resolution limit in the control

valve or variable speed drive assuming the

pulse amplitude is at least 5 times as large as

the suspected deadband and resolution limit.

This normally is the case if the amplitude is

> 5%. However, for valves designed for tight

shutoff, the backlash and stiction may be as

large as 10% requiring 50% amplitude.

You may want to check your pulse now to see

how excited you are about PWM opportunities.

You can read McMillan’s monthly Control Talk Column

printed in Control magazine.

http://www.controlglobal.com/voices/mcmillan-weiner/

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Getting the most out of valve positionersGreg McMillan

Today’s smart digital valve positioners have incredible capability and flexibility as to

tuning, performance and diagnostics. Here we look at how to get the most out of

these positioners by tuning and by making sure the valve assembly does not hinder

performance and gives the position feedback needed.

First of all, the positioner needs to know the actual position of the internal flow element

(e.g., plug, ball, or disk). “High Performance” control valves often have the lowest cost

and least leakage and often a straightforward compliance with the piping specifica-

tion for isolation valves. The appearance of a win-win situation is the root cause of poor

performance that often cannot be fixed by even the best valve positioner. The feedback

measurement is often on the actuator shaft. Since “High Performance” valves tend to be

rotary valves, there is backlash and consequently deadband in the linkages and connec-

tions that translate actuator movement into valve stem movement. Then due to the high

sealing friction particularly near the closed position from the plug, ball or disk rubbing

against the seal, there is considerable friction and a poor resolution. In many cases, the

valve stem may be moving but the actual plug, ball or disk is not. This stem windup

(twisting) may eventually cause the internal flow element to jump to a new position

overshooting the desired position. The smart valve positioner that is measuring actua-

tor shaft position doesn’t see what is really happening in terms of stem and internal flow

element position.



So what you need is a true throttling valve

and not an isolation or on-off valve posing

as a control valve. A true throttling valve

has a diaphragm actuator, splined shaft to

stem connections, a stem that is integrally

cast with the internal flow element (no

stem to element connections), and no rub-

bing of the element against the seal once it

opens. To achieve isolation you then install

a cheap low leakage on-off valve in series

downstream of the throttling valve. For pH

control, the on-off valve should be close to

the injection point to reduce reagent deliv-

ery delay upon opening.

Diaphragm actuators are now available

with much higher actuation pressures en-

abling their use on larger valves and higher

pressure drops. If you still need to go to

a piston actuator, the one with the least

sliding friction giving the best resolution

(best sensitivity) is the best choice (given

the reliability is good). Again, the posi-

tion shaft connections need to be splined

(keylock and pinned connections have a

surprising amount of play causing backlash

and shaft windup). The positioner feedback

mechanism must be properly adjusted to

give as accurate an indication of position

as possible.

Not all smart positioners have a good sensi-

tivity and sufficient air flow capacity. Spool

type positioners and low air consump-

tion positioners will cause long response

times for small and large changes in sig-

nal, respectively. Most tests done for step

changes in signal of 2% to 10% don’t reveal

a problem. High performance valves will be

lying to even the best positioners, making

diagnostics and supposed step response

capabilities invalid. For faster response on

fast loops, volume boosters should not re-

place positioners but be used on the output

of the positioner with its bypass cracked

open for stability.

Given that you have a good actuator,

A true throttling valve has a diaphragm actuator,

splined shaft to stem connections, a stem that

is integrally cast with the internal flow element

(no stem to element connections), and no rubbing

of the element against the seal once it opens.



throttling valve and positioner you are

still not home free until you tune the posi-

tioner. We know from PID control, that a

loop’s performance is only as good as the

PID tuning. In fact the Integrated Absolute

Error and Peak Error are functions of the

tuning settings. A loop with great valve,

process and sensor dynamics will perform

as bad as a loop with poor dynamics if the

controller is poorly tuned.

Positioners have traditionally been high

gain proportional only controllers. If a

high gain sensitive pneumatic relay is

used in the positioner, position control

can be quite tight since the offset from

setpoint for a proportional only controller

is inversely proportional to the control-

ler gain. The offset is also of little conse-

quence, since the effect is rather minor

and short term with the process controller

correcting the offset. What the process

controller needs is an immediate fast total

response. There are much larger nonlineari-

ties and offsets that the process controller

has to deal with. The original idea of cas-

cade control is to make the inner loop (in

this case the positioner) as fast as possible

by maximizing inner controller gain, which

means going to proportional or propor-

tional plus derivative control. Integral ac-

tion in the inner loop is hurtful unless we

are talking about a secondary flow loop

for ratio control or feedforward control.

The advent of smarter positioners has led

to much more complex control algorithms

that include integral action. The use of

integral action may make the valve step

response tests look better by the final posi-

tion more closely matching the signal. Not

realized is that the positioner gain has to

be reduced and that integral action in the

positioner increases the instances of limit

cycles. In fact, with the process controller

in manual (positioner signal constant) a

limit cycle will develop from stiction in the

valve unless an integral deadband is set.

Also, the increase in the number of integra-

tors in the control system means that the

process controller with integral action will

develop a limit cycle from backlash since

there are now two integrators. So here

we have the common situation where an

attempt to make appearances look better

we have created a problem due to lack of

a fundamental understanding. Many posi-

tioners now come with the integral action

turned on as a default.

A loop with great valve, process and sensor

dynamics will perform as bad as a loop with poor

dynamics if the controller is poorly tuned.



The solution is to omit integral action and

use the most aggressive gain setting. For

the Digital Valve Controller, this means

going to travel control instead of pressure

control. Overshoot of the setpoint is not a

problem as long as the oscillation quickly

settles out. Some overshoot helps in terms

of working through deadband and resolu-

tion limits faster and increasing the size of

the step seen by the positioner algorithm

with its sensitivity limits. In fact, a lead/

lag with lead time greater than lag time

on the input signal is sometimes used to

accomplish the same result. You should

not get hung up on the exact change in

position for a change in signal. For small

signal changes, the linearity due to resolu-

tion limits is going to look bad because

the resolution as a fraction of a small step

is large. The really important thing is that

the position changes quickly and the 86%

response time is fast. Positioners with

poor sensitivity and tuning may have a

response time that is an order of magni-

tude larger than possible. What the pro-

cess loop really wants is the manipulated

variable to respond quickly to its demands

and corrections. Also, for backlash, limit

cycles can be eliminated or at least the

amplitude reduced by a higher gain.

For much more on how to get good valve

rangeability and a sensitive and fast re-

sponse see the 5/01/2016 Control Talk Blog,

“Sizing up Valve Sizing Opportunities” and

the Control May 2016 feature article, “How

to specify valves and positioners that don’t

compromise control.”

Make sure this is not just talk. Put yourself

into a position to get the most out of valve

positioners.

What the process loop really wants is the

manipulated variable to respond quickly to its

demands and corrections.

http://www.controlglobal.com/blogs/controltalkblog/sizing-up-valve-sizing-opportunities/

http://www.controlglobal.com/articles/2016/how-to-specify-valves-and-positioners-that-dont-compromise-control/



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Maximum rangeability of linear valves

QI happened to read your article, “Rangeability of equal-percentage control

valves” (Sept. 2013, p. 102). It was informative and crystal clear as to what range-

ability for =% control valves means. This is the first time my mind was getting

clear on this subject as I was reading sentence by sentence. Thanks so much, but I also need

some of the same information for linear control valves.

I’m using a pressure-independent balancing and control valve (PIBCV), flow range 180 l/h to

1,300 l/h, with a proportional actuator. It’s a linear control valve. Valve authority is 100% and

DP across these valves is constant. Please let me have more information about the range-

ability of these PIBCVs and also comment on the maximum rangeability I can expect. Is

1:300 possible by any chance, or is even 1:50 possible or not? I appreciate your response.

Milind Pawar

[email protected]

ABefore talking about your PIBCV, I will discuss control valve rangeability and char-

acteristics in general. (This topic is covered in more detail in Volume 2, Chapter

4.13 of my handbook).

If in a particular control loop our goal is to obtain quarter-amplitude damping, stable control

will be obtained if the loop gain (Gloop = controller output/measurement input) is about

http://www.controlglobal.com/articles/2013/rangeability-of-equal-percentage-control-valves

http://www.controlglobal.com/articles/2013/rangeability-of-equal-percentage-control-valves



0.5. The loop gain equals the product of

the gains of the loop components. (Gloop =

Gprocess x Gsensor x Gcontroller x Gvalve).

Naturally, to obtain this loop gain on a fast

(high gain) process, the controller must be

tuned for low gain (wide proportional band)

and inversely, for a slow processes, the con-

troller gain has to be high.

One should use a linear control valve when

all the other loop components, including

that of the process, are more or less linear.

Note that a device or a process is linear if

their gains (% input/% output) are constant

throughout their ranges. For example, a

control valve is linear if its gain is constant

within its stroke range set by its rangeabil-

ity (Gvalve = maximum flow/100% stroke

= constant). Determining the best valve

characteristics for a particular application

can require complex dynamic analysis, but

for most common applications, my rules of

thumb are listed in Table I.

Valve rangeability is the ratio of maximum

to minimum controllable flow. The term

“controllable” is a matter of definition. I like

to limit the valve operation to a flow range

inside which the valve gain remains within

±25% of its theoretical gain. Figure 1 illus-

trates the rangeabilities of both a linear and

an =% valve. The dotted line #3 assumes the

theoretical gain of a particular linear valve

as 1.0, while the solid line #4 shows the ac-

tual gain of that linear valve. According to

my definition, the rangeability of that valve

is the ratio of maximum to minimum flows

within the flow range, where the actual

valve gain is within ±25% of the theoretical.

In this case, it is about 60%/3% = 20:1.

TABLE I: VALVE CHARACTERISTICS SELECTION GUIDE

ServiceValve characteristicpmax/ pmin < 2:1

Valve characteristic2:1 < pmax/ pmin < 5:1

Orifice-type flow Quick-opening Linear

Flow Linear Equal %

Level Linear Equal %

Gas pressure Linear Equal %

Liquid pressure Equal % Equal %

A TALE OF TWO VALVESFigure 1: Theoretical vs. actual characteristics of two valves tested by Les Driskell.

1

2

3

4

1 Theoretical gain characteristics of equal % valve2 Actual, inherent gain characteristics of equal % valve3 Theoretical gain characteristics of linear valve4 Actual, inherent gain characteristics of linear valve

2.5

2.25

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

0.125

Gain

1

2

3

4

10 20 30 40 50 60 70 80 90 100

% Flow

±25 of linear

±25 of =%

Smallvalve

Largevalve

VPC

Processcontroller

SP = 50%

Integral-onlycontrol mode

Gap action on error



Now, as to the rangeability of the PIBCV

valve of AB that you asked about. This

valve is a 3/4-in., flow-limiting valve used

in HVAC applications to balance water

distribution among many radiators. While

the supplier calls this valve linear, its full

stroke lift is only 0.2 in., so while it can

be linear within that distance, the design

is bordering on quick opening. This valve

is good for providing a limit of maximum

flow within an adjustable range of 5:1, but

I would not use it for closed loop control,

only for flow limiting.

For the 50:1 or 300:1 rangeabilities you

mentioned, you need at least two linear

valves in parallel, a larger and a smaller one.

In high-quality linear valves, you can obtain

a rangeability of 20:1 in each.

Béla Lipták

[email protected]

AYou can find the mathematical

derivation of the installed flow

characteristics of a control valve

in Carlos Smith and Armando Corripio’s

Principles and Practices of Automatic

Process Control, 1st edition, John Wiley &

Sons, N.Y., 1985, pages 147-152.

I have my own derivations of the same, fol-

lowing the method used by Smith and Cor-

rpio. I obtained the same graphs as the ones

published in Smith and Corripio, albeit I used

Matlab software. If you would like me to send

you this material, you can contact me.

I’ve read that “rangeability goes up to infin-

ity” as published by a major control valve

manufacturer. These needle-type valves

were put in water spray service in a desu-

perheater, attemperation application. They

all failed. The stems had to be replaced at

every shutdown during 2000-09, when I

was a control valve specification holder. I

hope these help. Thanks for your interest.

Gerald Liu, P. Eng.

[email protected]

AMy ISA book (Wade, H.L., Basic

and Advanced Regulatory Con-

trol, System Design and Appli-

cation, 2nd edition—3rd edition available

soon) describes two possible ways to

increase valve rangeability:

• Install a small valve and a large valve to

operate in parallel; or

• Install a small valve and a large valve to

operate in sequence.

No valve is perfect. Every valve has some friction

involved in moving the stem, and thus some

uncertainty of position and flow.



Small and large valves

operating in parallel

(Figure 2) depicts a

valve configuration

in which a small valve

and a large valve are

used in parallel. The

small valve is used for

moment-to-moment

control of the pro-

cess. The large valve

is operated by a valve

position control-

ler (VPC). Its measurement is the process

controller’s output. Or better yet, if a stem

position transmitter is available on the small

valve, then its output can be the measure-

ment of the VPC. The valve position control-

ler attempts to keep the small valve within its

operating range by increasing or decreasing

the large valve’s position whenever the small

valve approaches the upper or lower limits

of its range. The valve position controller is

an integral-only controller with a setpoint of

50%. It’s recommended that the control algo-

rithm should have a gap centered on either

side of 50% and within that gap, the effective

error is to be zero.

Reasonable limits for the gap are 20% and

80%. In this control system, the output signal

of the process controller always moves the

stem position of the small valve back to within

its range limits. This allows the large valve to

remain in its new position until the small valve

position reaches one of the limits of its gap.

The purpose of the

gap is to permit the

small valve to have

a reasonably wide

range of travel with-

out causing a “hunt-

ing” movement of

the large valve.

Small and large

valves operating in

sequence: If the pro-

cess turndown re-

quirements necessitate use of an equal-per-

centage valve characteristic (see Chapter

3 and Appendix A) with wider rangeability

than can be provided by a standard control

valve, then two equal-percentage valves,

one large and the other small, can be in-

stalled in parallel but operated in sequence.

The Cv’s of the valves should overlap, and

pressure switches or other logic devices are

provided to make sure that only one of the

valves will be open. Then the required Cv is

within the overlap range.

Also see Shinskey, F.G., Process Control

Systems, Application, Design and Tuning,

4th edition, pages 61-64 for more detail on

this approach.

Harold Wade

[email protected]

AThis is a great question and one

that I’ve considered for years.

The issue of valve rangeability is

COOPERATING ON THE RANGEFigure 2: Parallel valve operation for increased rangeability.

1

2

3

4

1 Theoretical gain characteristics of equal % valve2 Actual, inherent gain characteristics of equal % valve3 Theoretical gain characteristics of linear valve4 Actual, inherent gain characteristics of linear valve

2.5

2.25

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

0.125

Gain

1

2

3

4

10 20 30 40 50 60 70 80 90 100

% Flow

±25 of linear

±25 of =%

Smallvalve

Largevalve

VPC

Processcontroller

SP = 50%

Integral-onlycontrol mode

Gap action on error



often misunderstood and ignored. No valve

is perfect. Every valve has some friction

involved in moving the stem and thus some

uncertainty of position and flow.

You’ll need to consider just how you want

to define rangeability. It’s not as simple

as it might seem. Typically, it’s defined as

maximum flow/minimum controllable flow.

You can expect a control valve to affect

flow even at very small opening, but the

problem is that, since the uncertainty of

valve stem position in mm is more or less

constant over the stroke range, this be-

comes a large portion of the stem position

at low flows. This uncertainty of, say, 1 mm

of stroke is a much larger percent at low

flows than that same difference when the

valve is nearly full open.

If you have access to the ISA standards,

please look at ISA 75.25.01 and 75.25.02.

The first defines all the terms and dem-

onstrates how they work, and the second

provides the background.

The short answer is that all valves I ever saw

suffered the problem that there is some small

change in input signal that doesn’t cause the

valve to move at all. That small change might

not be identical for an increase in signal as for

a decrease in signal. In other words, a valve

is likely to have a “memory effect” in that the

state of the packing is not the same after a

move up as it is moving down. There also are

differences between a slow rate of change in

signal and a faster rate of change in signal.

In real life:

• A more powerful actuator is likely to posi-

tion the valve more accurately.

• Some packing materials have less fric-

tion, and thus allow better accuracy of

positioning. If you don’t really need high-

temperature packing, don’t use it. Graph-

ite packings normally have much greater

friction than most others.

• An eager operator with a wrench can

tighten a control valve packing gland

enough to prevent any motion and any

control. It can be valuable to check valve

action if control problems exist.

• All control loops with reset action will cycle

their control valves. You can’t escape that.

You can’t tune the controller to eliminate

that. You can slow the reset and slow the

reset cycle. You can reduce the proportional

gain and hide the cycling. It will not go away,

but it can be reduced with a better actuator/

positioner and low-friction packing.

I once did an ISA paper describing how to

analyze the required control loop perfor-

mance to estimate the control valve perfor-

mance that has to be specified. One of the

conclusions was the requirement to con-

sider how fast the process can change and

also how accurately the flow needs to be

controlled. Keep in mind that control can’t

be perfect; never accept the demand for it.

Cullen Langford

[email protected]

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Orifice flowmeter rangeabilityToday’s transmitters promise high turndowns, but other factors affect accuracy.

QI am measuring crude oil components (water, oil, gas) in a three-phase test sepa-

rator of capacity 3,000 barrels per day (BBD). I designed the orifice plates for

the oil and water to be of 1,500 BBD range. However, on actual testing, the oil per

well was found to be a maximum of 500 BBD, leading to inaccurate flow measurement. One

solution was to use multiple orifices to be swapped before testing each well, knowing ap-

proximately the capacity of the well in BBD and taking into consideration the orifice range-

ability. Or to use multiple transmitters.

I am thinking of using the existing orifice and transmitter (the existing transmitter range-

ability is 1:120) and changing the maximum range of the transmitter’s differential pressure

to approximately correspond to the well capacity, using the HART communicator. Will this

be a good solution to overcome the accuracy problem? Will the accuracy be as good as if I

used multiple orifices or transmitters?

Mohamed Amin, director, process control

Comex Commercial Co.

[email protected]

AIn the past, it was common practice to stack and switch transmitters or switch

orifice runs to increase rangeability while maintaining accuracy (Figure 1). This is

no longer necessary because the differential pressure (DP) cell rangeabilities have



increased to as high as 200:1, which pro-

vides flow turndowns as high as 14:1.

As to the total precision of the measure-

ment, one must also consider the errors

caused by temperature, pressure and den-

sity variations. Therefore, it’s important to

use smart transmitters that automatically

provide that compensation. Where the very

best accuracy is required, flow calibration of

the complete meter run (the orifice, assem-

bled with the upstream and downstream

pipe, including straightening vanes, if any)

is recommended. Facilities are available for

very accurate, weighed, water-based cali-

brations in lines up to 24 in. (61 cm) diam-

eter and even larger, and for a wide range

of Reynolds numbers.

Table 1 lists the rangeabilities and accuracies

of four transmitter design configurations.

The accuracy values given in Table 1 assume

that the fluid density is constant, the Reyn-

olds number exceeds 10,000 throughout

the range (including at minimum flow), the

flow is not pulsating, the orifice bore diam-

eter is correctly calculated, and the plate

is correctly installed. In that case, the total

measurement error is the sum of only the

error contribution of the orifice plate and

that of the transmitter. The orifice error is

less than 1% of the actual reading (AR). The

error contribution of a pneumatic transmitter

is about 0.5% of full scale (FS), and the error

contribution of a smart electronic transmitter

is about 0.1% FS.

For detailed coverage of all aspects of

% of maximum flow

Low range Mid range High range

2.0%

1.5%

1.0%

0.5%

0.0%0% 20% 40% 60% 80% 100%

% o

f re

adin

g er

ror

% of maximum flow

Low range Mid range High range

2.0%

1.5%

1.0%

0.5%

0.0%0% 20% 40% 60% 80% 100%

% o

f re

adin

g er

ror

DIFFERENTIAL PRESSURE ERRORTriple-stacking of 0.1%-full-scale transmitters can limit measurement error to 0.5% over a range of 8:1.

TABLE 1: TURNDOWNS AND ACCURACIES OF TRANSMITTERS AND SYSTEMS

Type of SystemRange-ability

Orifice plate er-

ror

Trans-mitter error

Loop er-ror, 10%

flow

Loop er-ror, 25%

flow

Loop er-ror, 33%

flow

Loop er-ror

50% flow

Loop error, 100% flow

Pneumatic 1) Standard trans-mitter 2) Two plates or

two transmitters

3:1 6:1

<1% AR <1% AR

0.5% FS 0.5% FS

NG NG

NG 3%

~3% ~3%

2% 2%

1.5% 1.5%

Electronic3) Standard trans-mitter 4) Smart trans-mitter

4:114:1

<1% AR <1% AR

0.2% FS 0.1% FS

NG2%

2% 1.5%

~2% ~1.5%

~1.5% 1.2%

1.2% ~1%



orifice-type flow measurement, refer to

Chapter 2.21 in Volume 1 of the 5th edition

of the Instrument and Automation Engineers’ Handbook.

Béla Lipták

[email protected]

AMy general comment is that many

of our instrumentation rules of

thumb are more than 40 years old

and completely invalid. It may be worth-

while to collect and revalidate these old

rules. Also, by solving some of the related

equations, one can determine the answers

to these types of questions.

Side issue: An erroneous concept is that

pressure and temperature (P,T) compensa-

tion is only used to calculate mass flow. As

shown by the equations below, P,T compen-

sation is (as the name implies) the means

to correct either mass or volume flow when

the temperature or pressure varies:

Flow (mass) = k*sqrt ( P*density)

Flow (volume) = k*sqrt ( P/density)

In evaluating the accuracy of orifice flow,

first consider transmitter accuracy and

rangeability. Modern transmitters have a

P rangeability of up to 200:1, which equates

to a flow range of almost 15:1. Modern trans-

mitters can detect the P with an accuracy

of <0.1% full scale (FS), and require recali-

bration about every 10 years. The old rule

that a second transmitter is to be used if the

flow range is greater than 3.5:1 was corect

when the old pneumatic P cells were used.

In determining the total flow measurement

error, transmitter accuracy is not the limit-

ing factor. The total error is a function of

many possible error sources.

On orifice installation (apart from the

straight run considerations), many orifice

plates are just sandwiched between two

flanges supplied by the piping group, and

the orifice plates are not properly centered.

Other installation error sources include im-

pulse tap diameters that are too big, orifice

tap location imprecise relative to the orifice

plate, and upstream and/or downstream pip-

ing is not concentric. The old British Stan-

dard BS 1042 (early 1960s) was probably the

best document to give a good understand-

ing of the accuracy constraints (I still refer

to it now in preference to the successor ISO

standards). An orifice installation of this type

used to be known as Class C, while if the

orifice was installed in carriers with honed

piping runs, that was called Class A.

Also, the Reynolds number is a key factor in

the orifice calculation, although if it doesn’t

vary much over the measurement flow

range, you don’t need to worry about it. The

steps in making the calculation are to first

do the orifice flow calculation at the various

flows of interest, then compare measure-

ments based on these various calculations

(using flow = k*sqrt( Px) where x is the

flow calculation point. If the generated



P is in an acceptable range (e.g. 50 to 400

in.WG), which depends on pressure drop

constraints of the installation, and if the

proper compensation is used (next para-

graph), then the orifice plate does not to be

changed.

The latest multivariable transmitters (at

least the Emerson one) does calculate

the Reynolds number (i.e. does the orifice

calcuation, or at least has a multivariable

compensation look up table) based on the

fluid properties and flowing conditions (e.g.

viscosity table based on temperature mea-

surement).

Simon Lucchini, CFSE, MIEAust CPEng (Australia)

Chief controls specialist, Fluor Fellow, safety systems

[email protected]

ATotal flow measurement accuracy

is affected by P, beta ratio, pipe

diameter, use of a flow computer,

and straight run distance. A 10:1 in DP range

is a real stretch for even a modern P trans-

mitter. Selecting different transmitters with

different calibrations addresses only the

transmitter uncertainty.

Installation effects increase with Beta ratio.

The orifice plates should be inspected and

measured. Also, one of the flow standards

should be followed. You can use the partial

differential uncertainty equations to find the

economical approach for improved accu-

racy. All of this is in the API or IEC literature,

both are good.

I’ve used a temperature-controlled en-

closure to reduce the temperature effect.

Some users insulate the meter run to stabi-

lize orifice and pipe temperature.

For a decision, the acceptable uncertainty

must be defined, as perfect accuracy is

never possible. If you can field-calibrate

the meter run at operating conditions, the

uncertainty would be greatly reduced.

Your decision will probably be clear after

determining the cost for the various options

you’ve listed versus the predicted uncer-

tainty with each.

Cullen Langford

[email protected]

AMy recommendation would be

to recalculate the pressure drop

for different flows using the ori-

fice size as the only non-changing variable.

This is the only way to minimize the error of

the measurement. If you calculate the flow

using, as fixed variables, the pressure drop

and the orifice size, the flow values will be

very irregular. It’s better to use the flows and

orifice size as fixed values, to arrive at the

predetermined pressure drops, which can be

used more easily during recalibration.

The biggest issue is that the crude oil den-

sity (specific gravity) must be constant,

otherwise the measurement will be in error.

Alex (Alejandro) Varga

[email protected]

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Flow of sandy condensate

QFlow measurement of hydrocarbon with sand? I want to measure hydrocar-

bon condensate flow where the condensate contains about 1% sand on a mass

basis.

How would this affect the measurement if a Coriolis or a vortex flowmeter is used? The sand

comes from the wells, and is collected and removed downstream of the flowmeter in the

liquefied-petroleum separator.

Azri Syahmi

[email protected]

AEither Coriolis or vortex can wear at a rate proportional to the percentage of sand

in the flow, and both meters could have short lives. Sharp-edge orifices are simi-

larly vulnerable. The choice is affected by flow rate and the pipe size.

I would worry about any Coriolis flowmeter with a relatively thin metal wall. Magnetic,

ultrasonic and flow nozzles have been recommended. With a sufficiently high flow rate, an

elbow meter will also work. Any DP meter might need a chemical seal or liquid purge to

avoid plugging.

Cullen Langford, P.E.

[email protected]



AI would love to tell you that you

can use a Coriolis flowmeter, since

it would be unaffected by sand

entrained in the flowing hydrocarbon, but

the resulting mass flow would be biased by

the sand. It’s almost impossible to instru-

ment/measure/control a poorly designed

process. Remove the sand first, then mea-

sure flow after the sand removal. Fix the

process first, then choose where you install

instrumentation.

Richard H. Caro, CEO, CMC Associates

[email protected]

AMost flow measurement equip-

ment, such as vortex and Coriolis,

would work quite well. Having

said that, here are the caveats:

1. You need to periodically inspect the flow-

meter internals to make sure abrasion has

not damaged the vortex-shedding block

or the tubes inside the Coriolis.

2. You need to verify periodically the type

of sand particles to make sure that

silicates or other hard rocks are not

increasing because this could affect the

abrasion risk.

3. You need to periodically verify that the

quantity of sand is not affecting the mea-

surement. You have to remember that the

solids displace the oil/water mixture and

can affect the final measurement.

4. Since you have sand, I would use vortex

meters, even if the measurement may not

be as precise, since they are cheaper to

replace.

Alex (Alejandro) Varga

[email protected]

AThis is a common measurement

problem in coker feeds (sand and

clay up to 5% to 10%) from col-

umn bottoms in oil refineries. If you can use

a solids collection pot before entering the

straight-pipe mass flowmeter, you can read

the flow reliably until solids appear in the

flow tube.

You’ll experience noise problems if using

target or vortex meters. The best choice

I would love to tell you that you can use a Coriolis

flowmeter, since it would be unaffected by sand

entrained in the flowing hydrocarbon, but the

resulting mass flow would be biased by the sand.



would be to select a mass flowmeter in-

stalled in a vertical pipe with a solids re-

moval arrangement. Periodic draining is

the simplest way to solve your application

problem. Good maintenance will enable reli-

able measurement.

Ram Ramachandran

[email protected]

AA Coriolis meter is not the first

meter that would come to mind

for a fluid with solids. It may

technically be able to measure the flow

with 1% solids, however, I would be very

concerned about erosion, particularly for

the U-tube-style meters. The only possibil-

ity would be the straight-type meters, but

I would recommend that you check this

with the vendors. In recent years, they’ve

improved the coil/tube drive technology

to better cope with energy losses due to

two-phase flow (gas/liquid). But the tubes

in Coriolis meters are thin, and can’t cope

with erosion or corrosion.

Vortex meters may handle a small amount of

solids, but this is not ideal (vortex are usually

for clean service). Check with vendors on how

much erosion of the bluff body is acceptable.

I would consider going back to old-school

venturi or wedge flowmeters, but not ori-

fice. It would be best to install in a flow-up

configuration (avoid trapping of solids). If

that’s not possible, then a wedge flowmeter

would be OK in horizontal flow with the tip

of the wedge facing down (to not trap the

solids). Since the wedge is for low Reynolds

numbers, you would have to check.

There are other things to consider for

avoiding plugging of impulse taps.

Simon Lucchini, controls specialist, fluor

[email protected]

A Coriolis meter is not the first meter that

would come to mind for a fluid with solids.

It may technically be able to measure the flow

with 1% solids, however, I would be very

concerned about erosion.



Piercing thick flow problemsFrom mud slurries to butter, tough measurement problems often call for clever solutions.

by Dan Hebert, P.E.

Difficult flow measurement problems require innovative solutions, ranging from us-

ing existing flowmeters in a unique way to developing entirely new technologies

because current instruments can’t handle the application. In this article, we’ll look

at several creative solutions to flow measurement issues.

Build a better Coriolis meter

Keith Simpson is the I&E controls manager for Continental Carbon (www.continentalcarbon.

com), which makes carbon black in Houston. Simpson recently helped develop a new pro-

cess for producing carbon black, but he and his colleagues couldn’t measure mass flow with

any existing products. They had to measure the mass flow rate of a feedstock coming out of

a preheater at 500-600 °F.

“At the time, no manufacturer made a Coriolis meter that would run at temperatures that

high,” says Simpson. “We tested several, burned them up in our pilot plant, and proved they

wouldn’t work.”

Simpson explained the problem to Micro Motion (www.micromotion.com), which developed

the DT Series Coriolis meter to address his issue (Figure 1). “The meter ran very successfully,

gave us the accuracy we required, and operated at the full temperature range that was nec-

essary, and we were able to begin manufacturing this very specialized product,” says Simp-



son. “Without Micro Motion, I don’t think

we’d have been as successful. We would

have had to do things differently, and we

wouldn’t have been as efficient.” [For a vid-

eo about this application, visit www.whymi-

cromotion.com/Asset/Video?assetId=217]

EXTREME ENVIRONMENTS“The difficulty of applying flowmeters to

most processes tends to increase when the

application entails relatively extreme oper-

ating conditions,” explains David Spitzer,

partner and co-founder at consultant

Spitzer & Boyes (www.spitzerandboyes.

com). “For example, a metal flowmeter is

usually better than a plastic flowmeter if

the fluid is hot. The flow of abrasive fluids is

usually better measured using an obstruc-

tion-less flowmeter. Flowmeters that can be

easily cleaned are usually preferred when

fluid can plug the sensing element.

“Applications that exhibit one of these

relatively extreme operating conditions

generally tend to eliminate consideration

of several flowmeter technologies. Further,

it can be downright difficult to find a vi-

able flowmeter that can measure accurately

when multiple extreme operating conditions

are present at the same time.”

In one application, Spitzer had almost the

same problem as Simpson—trying to mea-

sure hot fluids, this time in an incinerator.

The incinerator temperature was controlled

to maintain approximately 1,000 °C by mea-

suring and controlling flows of various si-

multaneously fired fuels, fume streams from

various process units and combustion air.

“The operators wanted a meter to measure

the flow of distillation residues to the incin-

erator, so they’d know the pipe was prop-

erly cleaned and not plugged,” says Spitzer.

“So the flowmeter had to measure liquids

and gases that could be hot or warm, plus

be rugged enough for the operator to occa-

sionally use a hammer and chisel to remove

solids from its body.”

In this case, the solution was a wedge

flowmeter. “The differential pressure that

a wedge flowmeter produces can mea-

sure liquids and steam,” says Spitzer. “The

flowmeter was fitted with chemical tee and

remote diaphragm seal connections that

eliminated impulse tubing, and located the

HOT FEEDSTOCK FLOW Figure 1: Continental Carbon uses a Micro Mo-tion high-temperature Coriolis flowmeter en-cased in a heating jacket to measure 500-600 °F feedstock for carbon black. Source: Continental

Carbon and Micro Motion



diaphragm flush with the flow. The opera-

tors could unbolt the diaphragm seals and

chisel out residue from the rugged all-metal,

wedge-flow element. A local switch was

installed to signal the type of operation,

residue or steam, so the control system

would know when to include the heat value

of the residue in the incinerator’s heat cal-

culations.”

CLOBBERED BY MUDUnlike the challenges of heat, Jason Nor-

man, a consultant and drilling fluids en-

gineer with Zaxxon Instruments (www.

zaxxoninstruments.com), had a clogging

problem with drilling mud, and he solved it

with plumbing and maintenance.

“We were working with a large independent

operator in the Texas Panhandle, using two

4-in. Coriolis meters on the mud pumps

plains. “We incorporated green cement into

the active mud system, which caused the

drilling fluid to gel up due to excessive wa-

ter incorporated into the system. We refer

to it as ‘clobbered up mud.’ We didn’t have

a meter bypass built into the plumbing,

which resulted in excessive downtime while

we waited for the crew to break apart the

meters to clean the inside of the tubes.

“The lessons we learned were to always

build in a meter bypass, so operations can

continue pumping during meter mainte-

nance intervals, and develop preventive

maintenance routines for operations that

can cause cold mud to gel inside the me-

ters.”

Denver Smart, vice president of oil and gas

marketing at Emerson Process Management

(www.emersonprocess.com), reports that

the growing trend to cut drilling costs de-

spite increased well complexity has prompt-

ed oilfield service companies to implement

more advanced measurement solutions for

drilling fluids (mud) management. “Us-

ing flowmeter technology to continuously

measure drilling fluid returns is a challeng-

ing application,” says Smart. “The various

oil-water-base slurries, changing physical

properties, potential for entrained gas and

The lessons we learned were to always build in a

meter bypass, so operations can continue pumping

during meter maintenance intervals, and develop

preventive maintenance routines for operations

that can cause cold mud to gel inside the meters.



the presence of rock cuttings significantly

impact the accuracy and reliability of flow

and density instruments.”

Coriolis flowmeters are widely used in

oilfields. Before Coriolis, methods for mea-

suring flow typically involved mechanical

paddle meters and/or pit-level measure-

ments. Both have issues with accurate and/

or timely flow measurement of the drilling

fluids return stream.

“For Coriolis technology, the measurement

performance at flow turndown ratios up

to 100:1 lets us use large-capacity meters

to prevent erosion and cuttings plugging

the flow line, while reducing backpressure

effects in gravity-feed applications,” Smart

explains. “Best practices have been estab-

lished to ensure full stream flow through the

meters under gravity feed, resulting in real-

time sustained measurement performance

and reliability. This provides a level of drill-

ing operational diagnostics to avoid well-

control events, reduced drilling efficiency

and higher operating costs.”

TWO FREQUENCIES BETTER THAN ONEOlder, 60-Hz magmeters have a difficult

time measuring the white and black liquors

in pulp and paper processes. These are par-

ticularly difficult in slurry applications due

to the presence of wood chips and other

pulp material.

“High-frequency, AC magmeters do a

good job of reducing slurry noise, but

they have poor accuracy and problems

with zero stability. DC magmeters can’t

be effectively used in this type of applica-

tion due to the presence of low-frequency

slurry noise,” explains Tom Figlik, field

instrument consultant at Yokogawa Corp.

of America (www.yokogawa.com/us).

“In this and similar applications, dual-

frequency magmeters combine the best

of high- and low-frequency meters. In

addition, dual-frequency magmeters use

both high 75-Hz frequency and low 6.25-

Hz frequency excitation. Dual-frequency

excitation superimposes high frequencies

on low frequence.”

Before Coriolis, methods for measuring flow

typically involved mechanical paddle meters and/

or pit-level measurements. Both have issues with

accurate and/or timely flow measurement of the

drilling fluids return stream.



MORE PRESSURE FOR PAPERLikewise, a German pulp and paper manu-

facturer wanted to increase steam pressure

at the inlet of the dryer section of a paper

machine to increase production. The com-

pany measured steam flow to determine the

proper amount of specific energy to apply

for optimal paper production. The exist-

ing orifice-plate, differential pressure (DP)

flowmeter wasn’t suited to this high-pres-

sure service because it caused a substantial

reduction in pressure.

As a result, the papermaker replaced its old

meter with a Rosemount 3095MFA An-

nubar flowmeter (Figure 2). Its calculations

showed low permanent pressure loss versus

an orifice-plate meter, and minimal modifi-

cations were needed to the existing piping.

The new flowmeter provided a pressure-

and temperature-compensated measure-

ment in one package.

After replacing its old flowmeter with the

Annubar, the paper machine achieved

record high paper production due to

increased pressure in its steam heads.

Consequently, the paper company also

achieved higher production rates and ef-

ficiencies.

However, one replacement project doesn’t

mean orifice-plate flowmeters are no lon-

ger useful, even though they require rela-

tively long straight-pipe runs upstream and

downstream of the meter, which is an issue

in many retrofit applications. “In the nuclear

industry, additional regulations following

the Fukishima disaster are calling for more

flow measurements in existing pipes for

parameters like non-safety-related cooling

water,” says Wally Baker, pressure content

marketing manager at Rosemount (www.

rosemount.com). “When flow measure-

ments are required on existing infrastruc-

tures, it can be challenging to meet best

practices for installation, including require-

ments for straight-run piping for orifice-

plate measurements.

MORE STEAM, MORE PAPERFigure 2: To increase steam pressure at the inlet of the dryer section on a paper ma-chine, a German paper manufacturer replaced orifice-plate differential pressure (DP) flow-meters with a Rosemount 3095MFA Annubar flowmeter, which provided a pressure- and temperature-compensated measurement in one package, and spurred record high paper production. Source: Emerson Process Management



“Fortunately, conditioning orifice plate

(COP) technology enables users to perform

these measurements because it doesn’t

require the traditional four pipe diameters

of upstream pipe. A COP only requires two

pipe diameters upstream to make an accu-

rate flow measurement. This enables users

to make these newly required flow mea-

surements with little or no piping modifica-

tions,” concludes Baker.

MEASURING BUTTER FLOWNathan Hedrick, flow product market-

ing manager at Endress+Hauser (www.

us.endress.com), reports having a customer

that needed to measure flow of butter as a

part of a large batching process for making

baked goods. The butter had to stay below

40 °F, but at that temperature it no longer

flowed as a liquid, was highly viscous, and

trapped a lot of air. “The customer was us-

ing a Coriolis mass flowmeter to measure

flow,” notes Hedrick. “Initially, the readings

were unrepeatable, inaccurate and unreli-

able because of the entrained air.”

Hedrick adds that Endress+Hauser worked

with its bakery customer to make this very

difficult application work. “We looked at

the overall process, and made recommen-

dations on how they could improve the

process to reduce the amount of air,” he

explains. “We installed a perforated plate to

put backpressure on the meter, thereby re-

ducing air pockets. In addition, we provided

a Promass 83I full-bore, single-tube, Coriolis

mass flowmeter to minimize pressure loss.

Digital signal processing capabilities in the

electronics of the meter enabled successful

results despite the challenging conditions.”

These and other challenging flow measure-

ment problems can be solved by calling in

an independent consultant or a flowmeter

supplier. Consultants aren’t beholden to any

supplier, and can thus recommend the best

solution from a wide range of choices, but

they may not have detailed knowledge of

each flowmeter type. Meanwhile, suppliers

with a wide range of flowmeter types can

be another good source for solutions. They

have the ability to produce custom prod-

ucts, and can also offer innovative ways to

apply their off-the-shelf offerings.

Challenging flow measurement problems can be

solved by calling in an independent consultant or

a flowmeter supplier. Consultants aren’t beholden

to any supplier, and can thus recommend the best

solution from a wide range of choices

STATE OF TECHNOLOGY 2017 FLOW MEASUREMENT

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