16  Pumps, Compressors and Process Components 2012

Pumps

Double hose-diaphragm pumps

To a large extent, the efficiency of wet air oxidation units depends on the

availability and reliability of the pumps that feed the wastewater into the

oxidation reactor. As a general rule, positive displacement pumps are used

for this purpose. Apart from maximum availability, these units have to

ensure minimum pulsation and maximum accuracy of the feed range.

With a high level of redundancy and state-of-the-art diagnostic systems,

double hose-diaphragm pumps are meanwhile well-established in this

process. At the heart of this pump are two hose-diaphragms, although the

pump only requires one to be fully operational. Both hose-diaphragms are

arranged one inside the other and subject to hydraulic actuation. The

contraction is similar to the peristalsis of the human intestine. Hose-

diaphragms provide for a linear flow path of the wastewater throughout

the pump so that it is in contact with the inner hose-diaphragm and check

valves only. Check valves are virtually the sole wearing parts of this pump

design. Double hose-diaphragm pumps are designed without flat dia-

phragms, the typical characteristic of traditional piston diaphragm pumps.

This allows for a slim, cylindrical shape of the pump heads, small footprint

and high strength. Double check valves ensure maximum feeding accuracy

(reliability), which is indispensible for a proper balance within the reactor.

Wet oxidation is based on the principle of hydrothermal oxidation. Within this system  the  wastewater  is  subject  to strong heating prior to being pumped into  the  reactor  under  high  pressure, where the oxidisation process for mi-neralisation  of  organic  material  is  in-duced by means of pure oxygen. Wet oxidation procedure is followed by de-watering and drying. Residuals are re-duced to a minimum and disposed as cover product for land fills, for example. The process offers an environmentally friendly  alternative  to  traditional  in-cineration  methods  and  is  particular-ly conducive to toxic wastewater with high contents of organics and sludge, as  well  as  for  such  cities  that  cannot consider  land  disposal  of  sludge  as  a possible solution.

Wet oxidation is classified as par-ticularly  environmentally  friendly,  be-cause  all  components  of  the  waste-water  sludge  are  either  subject  to recycling,  reclamation  or  emission  to the  environment  without  negative impact. First systems of this kind were developed in the USA by F. J. Zimmer-mann  about  1940  with  the  objective of deriving synthetic vanilla extract di-rectly from digestion systems of pulp and paper industries by means of par-tial oxidation of lignosulphonic acid. A patent  for  the  Zimmermann  process was filed in the year 1950, but only ap-proximately  10  years  later  it  was  ap-plied in the field of municipal sewage sludge treatment for the first time.

Process and application

Meanwhile,  well-known  manufactu-rers offer different systems world wide, which  likewise  operate  at  different pressures  and  temperatures.  Oxida-tion systems that use air as oxidation medium and operate at tempera tures from  150°C  to  320°C  and  pressures from 10 to 220 bar are  referred to as 

Mineralisation and minimisation of wastewater sludge by wet air oxidationHydraulically actuated reactor feed pumps

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wet  air  oxidation.  Higher  tempe-ratures  require  accordingly  high-er  pressure  to  maintain  the  fluid phase within the system.

Supercritical  wet  oxidation calls for pressures of 250 bar plus and requires heating of the waste-water  to  temperatures  beyond 400°C.  Under  such  conditions, the  wastewater  to  be  treated  is in  a  supercritical  state  between gas  and  fluid,  whereby  organic matters  are  completely  soluble. By  means  of  injection  of  oxygen all  organic  substances  are  imme-diately  oxidised  so  that  almost nothing but water remains, which is  either  discharged  without  im-pact  on  the  environment  or  re-used as industrial water. Residual mineral, metal and similar materi-als are returned for recovery.

fected by means of a heating unit located  between  pump  and  re-actor. The  oxidising  agent  is  oxy-gen that either consists of air, an enhanced  oxygen  source  or  pure oxygen.  Oxidation  efficiency  de-pends  on  temperature,  oxygen partial  pressure,  residence  time and  oxidisability  of  the  organics that  are  included  in  the  waste-water. Another key criterion is the proper dosing ratio of sludge and oxygen. 

Working principle of hermeti-cally sealed reactor feed pumps in the process

High-pressure  feed  pumps  take a key position within the system. In  order  not  to  exceed  or  fall  be-low  the  required  sludge  volume, 

Fig. 1: Wet air oxidation – Simplified process diagram

However,  all  systems  have  one feature in common: Ranking fore-most  in  all  systems,  there  are high pressure pumps that provide for  the  feeding  of  the  thick ened sludge  into the oxidation reactor. In  general,  the  chemically  condi-tioned  sludge  is  of  toxic,  aggres-sive nature and not only contains typical  solids,  but  likewise  fibres. By  means  of  high  pres sure  dia-phragm pumps the sewage sludge is fed into the reactor. Whilst tradi-tional  incineration  plants  include oxidation  in  gaseous  phase,  wet oxidation applies oxidation of or-ganic  matters  in  liquid  phase  at temperatures between 125°C and 320°C,  in  order  to  avoid  evapora- tion of water. Sludge heating is ef-

the  feed  volume  must  not  differ from the specified nominal value. For  this  duty,  diaphragm  pumps have proven their effectiveness in testing  phases  of  pilot  plants  for many years.

However, the suitability of tra-ditional piston diaphragm pumps is  limited  when  it  comes  to  the handling of aggressive or abrasive products. Solids that settle in the lower  pump  area  between  dia-phragm  and  clamping  ring  may result  in  early  diaphragm  failure. In  the  event  of  a  diaphragm  fail-ure  with  such  pump  types,  the product  breaches  through  and thus  contaminates  the  hydraulic control  area  and  sliding  seals  as well  as  the  cylinder  face,  which 

18  Pumps, Compressors and Process Components 2012

Pumps

Double hose-diaphragm pumps

may  create  considerable  costs  for cleaning  and  subsequent  repair  of pump and gearbox.

Hose-diaphragms

With  hose-diaphragm  pumps,  the traditional  diaphragm  principle  has been modified into a flexible double hose-diaphragm,  which  –  similar to  the  traditional  piston  diaphragm pump – is likewise actuated by a hy-draulic fluid and provides for the dis-placement action. The rotary driving motion  of  the  pump  motor  is  con-verted  into  reciprocating  action  of the crosshead by means of the crank drive. The crosshead  is connected to the piston or plunger, respectively. By means  of  hydraulic  fluid  the  piston actuates  a  pair  of  redundant  hose-diaphragms, which are arranged one inside  the  other.  They  represent  the heart  of  this  pump.  Simultaneously, they  create  double  hermetic  sealing from  the  hydraulic  drive  end  of  the unit. Utilisation of flat diaphragms is fully abandoned. 

The  particular  advantage  of  this design is the linear flow path, so that it is especially conducive to the hand-ling  of  aggressive,  abrasive  and  so-lid  carrying  fluids.  The  unit  is  even capable  of  handling  wet  oxidation sludge  with  minimum  wear.  Sedi-mentation  in  the  diaphragm  clam-ping  area,  as  experienced  with  tra-ditional  piston  diaphragm  pumps, 

does  not  occur  with  double  hose-diaphragm  pumps.  With  a  great number of technical and commercial benefits, hydraulically actuated hose diaphragm piston pumps and double hose-diaphragm  pumps  are  there-fore  well-established  in  the  field  of wet oxidation systems. 

Low-wear design

Also  where  lifetime  is  concerned, hydraulically  actuated  hose-dia-phragms stand above traditional flat diaphragms.  For  this  reason,  hose-dia phragms are not considered as ty-pical wearing parts. Unlike peristaltic 

hose  pumps,  the  hose-diaphragms in  double  hose-diaphragm  pumps are not compressed or kneaded, but with every stroke merely complete a movement  comparable  to  a  human vein,  ensuring  particularly  long  ser-vice  life.  Simultaneously,  the  pair  of hose-diaphragms  create  double  her-metic sealing of the hydraulic actua-tion system and all sensitive compo-nents  from  the  wet  end  of  the  unit.  The medium only comes into contact with the interior of the primary hose- diaphragm and check valves, and not (as  in case of  typical diaphragm pis-ton pumps) with the often extreme-ly  complex  pump  cover  and  pump head. For this reason, the suction and discharge check valves are to be con-sidered as the sole real wearing parts.

Check valves

When  it  comes  to  the  handling  of wastewater  sludge,  especially  the check valves of the pump play a deci-sive  role.  Particularly  when  it  is  the question  of  aggressive  fluids  with fibrous components, it is essential to take precautions in terms of minimi-sation of valve wear and to avoid valve blockage by solid and fibrous matters in the planning stage already. 

Against  this  background,  double hose-diaphragm pumps that are de-termined for service in wet oxidation systems  are  provided  with  double check valves. The reason for this spe-

Fig. 3: Double hose-diaphragm pumps – Aix en Provence Sewage Treatment Works

Fig. 2: Hose diaphragm piston pumps – Brussels North Sewage Treatment Works Fig. 4: Double hose-diaphragm pump

Pumps

Double hose-diaphragm pumps

cial  equipment  is  not  attributed  to the  apprehension  that  single  valves might  not  provide  efficient  sealing, but  rather  to  the  overall  redundant character  of  these  pumps.  If,  in  the short  term,  a  particle  gets  jammed between  the  ball  or  cone  and  the valve seat resulting  in valve  leakage, the  second  valve  ensures  effec-tive  sealing,  thus  preventing  medi-um back-flow and a resulting loss of  volume.

Since reactor feed pumps require a particularly high degree of continu-ous flow, double check valves provide for  an  important  additional  safety factor. Nevertheless, double valve de-sign will not result in higher cost for wearing  parts.  Since  valve  wear  is  a function of  the differential pressure, wear  of  double  valve  assemblies  is accordingly halved.

All  check  valves  of  double  hose-diaphragm  pumps  are  individually customised in terms of choice of ma-terial as well as with respect to flow velocity  and  flow  geometry.  In  any case, the achievement of utmost life-time  is  paramount  when  designing the  valves.  Moreover,  all  parts  that are  theoretically subject  to wear are individually detachable without prior removal of adjacent elements.

Ball  valves  have  favourable  flow characteristics  and  are  self-cleaning. They are distinguished by almost op-timum Cd values and preferably used where media with high solid concen-trations or viscosities are to be pum-ped. With its very easy to install and maintenance-friendly design, the du-rable  pump  valve  design  clearly  sur-

passes similar constructions. The cas-settes  are  hinge-mounted  so  that the complete valve unit is removable within  a  very  short  time  without prior dismantling of piping and valve trims.  Valve  seats  and  balls,  by  way of example, are easily replaceable by swivelling of the valve casing.

Ball  valves  of  varying  materials are  available  with  ball  diameters up  to  300  mm  and  pressures  up  to 400 bar. However,  the wear  rate not only  depends  on  working  pressure and  particle  size,  but  most  notably on  the  degree  of  pressure  pulsa-tions.  Subject  to  the  individual  de-signs,  positive  displacement  pumps are  associated  with  most  differen-tial  pulsations. The  deviation  from a constant  volume  flow  is  reflected  in the kinematic irregularity. 

The delivery characteristic of a single- cylinder pump is approximately sinus-oidal.  When  comparing  single  and double-acting  single  and  multiple   cylinder  pumps,  it  is  obvious  that the  kinematic  irregularity  coefficient is  considerably  smaller  in  case  of  an uneven  number  of  cylinders  than  for an  even  number.  Preference  is  there- fore given to the realisation of pumps with  an  uneven  number  of  cylinders. For  this  reason,  the  majority  of  hose-diaphragm pumps are of single-acting triplex design. 

For  triplex  pumps,  the  kinematic irregularity  amounts  to  23.0 %  (see Fig.  6),  as  compared  to  32.5 %  of double-acting two cylinder pumps (see Fig.  7).  Special  pulsation  dampeners (Pulsorber)  allow  for  the  reduction  of the residual pulsation to 0.5 % p to p.

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Fig. 5: Swivelling of double check valves (suspension valves) without removal of piping

20  Pumps, Compressors and Process Components 2012

Pumps

Double hose-diaphragm pumps

Condition monitoring

The primary task of pumps in wet oxi-dation systems is availability. Unsche-duled downtime has to be avoided by all means. For this reason, it  is essen-tial  to  recognise  even  the  merest  in-dication of wear at an early state and to properly interpret the signals of in-stalled condition monitoring sensors.  

Whilst  mechanically  actua-ted  dia phragm  pumps  typically  re-quire preventive or corrective mainte-nance  after  occurrence  of  the  failure, double  hose-diaphragm  pumps  with reasonable  redundancy  offer  an  ideal 

Fig. 6: Delivery flow of a single-acting three-cylinder pump

starting  basis  for  predictive  mainte-nance. For this purpose they are provi-ded with an overall diagnostic system for  permanent  condition  monitoring of essential components and parame-ters (see Fig. 8), such as:

Automatic valve diagnostics for early recognition of wear in delivery valvesSo far, failure or leaks of delivery valves became  apparent  only  by  means  of exceeding admissible operating condi-tions, for example when the specified flow  rate  of  the  pump  could  no  lon-ger be reached. In such case, the actu-al cause for the loss of volume had to 

be determined with great effort and in many cases removal and inspection of all  check  valves  was  required.  Loss  of efficiency during the formation of the damage was accepted.

For  the  timely  detection  of  wear of  delivery  valves,  hose-diaphragm pumps  are  provided  with  a  tailored Valve Performance Monitoring System –  FVPMS  (see  Fig.  9).  The  measure-ment principle is based on the analysis of the inherent noise characteristic of the valves and is capable of identifying leaks  in a very early state, even  if  the loss of output is still less than 1.5 %. 

Fig. 7: Delivery flow of a double-acting two-cylinder pump with double-sided piston rod

Fig. 8: Double hose-diaphragm pump in quintuplex configuration with suction and discharge pulsation dampeners and diagnostic systems

Fig. 9: Condition monitoring of valves

This measurement principle takes ad-vantage of the effect created by onset of  leakage  in  delivery  valves,  namely the  partial  reduction  in  vapour  pres-sure  of  the  pumpage.  This  undesired 

Pumps, Compressors and Process Components 2012  21

throttle point creates a flow cavi-tation  with  a  typical  sound  spec-trum  and  simultaneously  pro-vides  a  measurement  value  for the  leakage  about  to  occur  in the  valve. The Valve  Performance  Monitoring  System  allows  for  a noticeable  increase  of  operating reliability  and  availability  of  the pumps, since incipient wear of de-livery  valves  is  precisely  detected at  an  early  stage.  This  does  not only avoid loss of energy, but also allows  for  specific  forward  plan-ning of service and repair. 

Hose-Diaphragm Guard (HDG)At  the  heart  of  double  hose-dia-phragm pumps are two hose-dia-phragms arranged one inside the other,  although  the  pump  only requires  a  single  one  to  be  ope-rational.  The  primary  hose-dia-phragm  that  is  in  contact  with the  product  is  hydraulically  cou-pled  to  the  secondary  hose-dia-phragm  that  faces  the  hydraulic fluid. The intermediate space bet-ween  both  hose-diaphragms  is unpressurised and ends  in a cen-tral transfer point. For this reason, condition monitoring of hose-dia-phragms does not require conduc-tivity  probes,  as  in  case  of  piston diaphragm  pumps.  Diagnostics for hose-diaphragms is rather en-sured  by  means  of  pressure  sen-sors,  pressure  gauges  or  contact pressure  gauges.  In  the  event that one of the hose-diaphragms 

leaks  or  fails,  either  product  or actuation fluid will penetrate into the  unpressurised  intermediate space.  The  resulting  build-up  of pressure  is  fed  to  the  condition monitoring system, which in turn provides  a  signal  with  manifold processing  options.  Nevertheless, operation can be maintained with a single hose-diaphragm until the system  allows  for  shutdown  and repair. The second one will ensure that  the  product  neither  comes into contact with the pump casing nor leaks out.

SuctionGuard (SG)Unrestricted  inflow  at  an  appro-priate  inlet  pressure  is  essential for  trouble-free  operation.  For  re-liable supervision of suction pres-sure, measuring is made at points where the measuring instrument is separated by a large-scale elas-tic separating diaphragm.  

TempGuard (TG)Supervision of hydraulic and gear-box oil temperature is carried out by  means  of  PT  100  temperature sensors.

Touch panelsFor  early  detection  of  faults  and with the objective of ensuring ma-ximum availability, the redundant nature of double hose-diaphragm pumps  can  additionally  be  sup-ported  by  the  provision  of  touch panels  (see  Fig.  10).  Touch  pa-

nels, which are integrated into the control  cabinet,  give  the  pump  a transparent character and provide the  operator  with  information on  current  operating  parameters and the condition of fundamental parts. Bus systems link the touch panel  to  local  process  control, whereby  PROFIBUS  (Process  Field Bus)  provides  best  conditions  for communication  and  control  of sys tem frequency converters, PLC, touch panels and the Valve Perfor-mance Monitoring System. 

FelWebGuard (FWG)The  system  is  linked  to  the  In-ternet.  In  the  event  that  actual values  differ  from  the  program-med  nominal  values,  the  system will  email  an  according  notifica-tion to a service technician. In ad-dition,  a  safe,  bidirectional  VPN conduit can be set up, which pro-vides for remote access to the con-trol unit of the pump. The system not only allows for higher availa-bility and productivity, but also for Fig. 10: Touch panel for simultaneous control of two triplex pumps

a reduction in service costs. All cri-tical  parameters  are  displayed  by means of traffic light logic.

Equipped  with  state-of-the-art  mechanical  and  electronic components  double  hose-dia-phragm  pumps  are  most  suited for  highest  demands  in  wet  oxi-dation systems, even for supercri-tical systems that require working pressures of 250 bar plus. Cylindri-cal pump casings of double hose-diaphragm pumps not only allow for  a  smaller  footprint  than  ca-sings of flat diaphragms, but addi-tionally provide for higher rigidity with excellent hydraulic efficiency up to 500 bar.

References

1)  Water  Practice  &  Technology Vol 5 No 12) Veolia Water – Athos

Author: Heinz M. NägelManaging Partner ofFELUWA Pumpen GmbH

Pumps

Double hose-diaphragm pumps

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