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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
Heinz M. Nägel
www.ifat.de
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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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