Protecting Progressing Cavity Slurry Pumps - Onyx …

1 1 Protecting Progressing Cavity Slurry PumpsA Progressing Cavity pump consists of a single helix metal rotor that turns inside a doublehelix rubber stator. This forms a series of pockets that traverse the length of the pump,gently pushing fluid from the suction to the discharge in a smooth, pulseless 1 Progressing Cavity pumpNormal direction of flow through the pump is towards the courtesy Netzsch Inc, NEMO Pump DivProgressing Cavity Pumps are unmatched in their ability to transport viscous, abrasive,and shear sensitive fluids. They can pump exotic solutions such as shrimp and brinewithout crushing or tearing the product. They routinely handle sewerage sludge, ceramicslurry, wax, chemicals, confectionery ingredients, grinding compound, sand, cement,grout, putty, pulp, ground meat, pigments, glue, paste, grease, paint, lime Slurry , jelly, precise internal geometry and minimal back flow make them accurate meteringpumps, eliminating the need for a flow meter.

2 You can deduce volumetric flow rate byadding a tachometer pickup to any rotating part of the pump and motor operate reliably, however, these Pumps must stay within certain operating they are a true positive displacement pump, discharge pressure theoretically spikesto infinity if the discharge is blocked. In reality, the motor stalls under these conditions,but usually not until after the flange bolts stretch or discharge piping minimize back flow, there is an interference fit between the metal rotor and the rubberstator, so these parts rub against each other during normal operation. The pump dependson the fluid stream to carry away the heat generated by friction. If the pump runs dry, thestator overheats until the rubber melts or scorches. At this point the ruined stator must Pumps handle difficult fluids that make it particularly challenging to monitor theprocess to prevent against damage from over pressure requires the addition of a pressure gauges and switches are prone to clogging or plugging when pumping the kindsof fluids usually handled by this type of pump.

3 Protecting gauges and pressure switches2with conventional diaphragm seals buys some time, but these devices become pluggedthemselves within a few days of most successful approach to date has been to use an annular ring seal, usuallyreferred to as an isolator ClogsDiaphragm SealWaffer RingIsolator is notAffected 2An Isolator Ring sandwiched between flanges transmits pressure to gauge or pressureswitch without cloggingAn isolator ring consists of a rubber 'inner tube' clamped in a steel ring. The assemblyfits between flanges in the process instrument oil behind the rubbermembrane transmits pressure to the gauge or pressure switch. The motion of the processfluid continuously cleans the inside of the ring the pump against damage from run dry condition is trickier; several methodshave are used, each with advantages and METER: An obvious method is to install a flow meter coupled to a signal flow falls below a certain threshold, electrical contacts open, stopping the pump.

4 Thisis straight foreword and reliable, albeit expensive. The only flow meters that can handlethe fluids typically conveyed by Progressing Cavity Pumps are magnetic or sonic meters,which cluster near the high end of the cost MASS DISPERSION:Thisdevice monitors flow by measuring the rateof heat dissipation from a probe in the and a heating element. Onethermocouple is next to the heatingelement, the other is some distance in the temperature of the no fluid motionaround the probe, the thermocouple closestto the heating element registers a highertemperaturethanthereferencethermoc ouple. When fluid flows past theprobe assembly, it dissipates heat from the'hot' thermocouple, so temperature at thisthermocouple measuring thedifference in temperature at these twopoints, the electronic circuit determines ifthere is sufficient flow thermal mass dispersion device is morecost effective than conventional flowmeters, but is subject to certain that build up a coating can insulatethe probe, degrading fluids can erode the probe to thepoint where it no longer 4 The Thermal Mass Dispersion DeviceThese devices can be difficult to calibrate because when a Progressing Cavity pump runsout of liquid, it becomes an air compressor!

5 Since thermal dispersion devices measure airflow as well as liquid flow, configuration and calibration can be difficult. For example,you can't test this device by simply turning the pump off; you have to force the pump torun dry to produce an air stream in the pipe to check if it is properly detecting fluid places the stator at DETECTORS: Depending on pipe size, cost is usually higher than athermal mass dispersion device. These are installed on the suction side of the do not monitor flow; they monitor the presence or absence of fluid in the device consists of an insulated ring sandwiched between flanges on the pump principle element in this device is a metal plate lining the inner circumference of thering. A layer of electrical insulation separates this plate from the process fluid.

6 Themetal plate functions as one side of a capacitor circuit; the fluid in the pipe functions asthe other fluid in the suction pipe, capacitance of the circuit is high. If the pipe is empty,capacitance of the circuit is low. An oscillator applies a reversing polarity signal to theplate. The circuitry measures the dampening effect of the capacitive plate and infers thepresence or absence of fluid within the boundaries of the coating or build-up on the inner surface of the device affects the reading, but a simplere-calibration compensates fro a buildup to " or more without adverse effects. Theyare immune to false reading caused by air flow during run dry conditions because they donot detect flow, they detect the presence or absence of material within the boundary of MONITOR: Another run-dry protection device is a thermocouple monitorburied in the stator.

7 A small hole is drilled through the stator tube and a thermocouple isinserted. As the rotor turns inside the pump, it rubs across the thermocouple for a portionof each revolution, allowing the thermocouple to sense the temperature of the rotor. Asignal relay amplifies and monitors the output from the thermocouple. If the pump runsdry, friction heats the rotor and stator. The thermocouple detects this temperature riseand triggers a signal to stop the advantage to this approach is low cost. However, fluid temperature fluctuations cantrigger false alarms. Conversely, cold ambient temperatures can delay hole involved prohibits the use of this device in sanitary, food, or high fluids will affect response time. Non-lubricating fluid such as lime Slurry willelicit a fairly fast response, but fluids which exhibit higher lubricity such as polymerflocculent may take several hours to evaporate.

8 The resulting gradual temperature risemay take several hours to reach the set point on the alarm relay. By that time the statormay have suffered substantial MONITORS infer the presence or absence of flow by noting changes in electricpower by the pump motor. However, the power dip caused by run dry conditions is shortlived and difficult to capture. The reason is that running dry causes rapid heat build fluid film lubricating the interface between the stator and rotor quickly evaporates, sofriction increases within seconds after loss of fluid. Soon, he heat build up in the statorcan causes the rubber to revert, making it gummy, and further increasing MONITORSP ressure monitors use pressure toinfer flow conditions. This method issimple and straight forward with theadvantage that you can monitor highpressure with the same displacement Pumps shouldalways be fitted with over operated against aclosed discharge (dead-headed) apositive displacement pump builds uppressure until a system componentfails.

9 The motor fuses may blow, butit is equally probable that a pipefitting will switches are more reliable onslurries than pressure relief valves andeasier to reset than rupture manufacturers offer controlboxes to interface with the pressureswitch and isolator ring 5 Typical isolator ring with control courtesy onyx Valve CoThese boxes incorporate local push button control stations and include a timer to allowthe pump time to prime itself each operating cycle. Some of these boxes incorporate sealflush controls as well. To use this approach, it is necessary to understand the differencebetween static pressure, friction pressure, and total pressureresults from the pipe being filledwith liquid and is present even when the pump isidle. It isnotinfluenced by pipe size, number offittings,or pressure isdetermined solely by fluid density and differencein height between the pressure switch and theoutlet of the the example in figure 6, the outlet of thedischarge pipe is 12 feet above the gauge, sostatic head is 12 feet, which exerts 5 psi pressureresults from the flow of liquidthrough a pipe and is present only when the pumpis running.

10 It depends on flow rate, size andlength of pipe, number of fittings, and pressureis the combination of static andfriction pressure is observeddirectly by reading the gauge on the psisuctiondischargefig head is the pressureresulting from the weight of theliquid in the the pump is idle, the gauge shows staticpressure. When the pump is operating with flowpresent the gauge shows total the example in figure 7, total pressure is run dry protection, set the low pressureswitch midway between the static and our example the correct setting forthe low pressure switch is 15 the pump is running correctly the lowpressure switch signals that flow is present. Ifthe pump runs dry and flow stops, the pressurefalls back to the static pressure.