Metering Pumps Trouble-Shooting Guide

Metering Pumps Common Start-Up Issues

Diaphragm PumpMost metering pump start up problems can be attributed to improper installation.

The considerations, rules of thumb, and suggestions listed below will insure proper selection and installation of a chemical feed pump or chemical feed system.

  1. Consider the application and level of quality. Is the unit for an apartment/school/hospital where economy is an important consideration? Or is the unit for an industrial plant/waste treatment facility/refinery/power plant where ruggedness and additional features are required? Is it a hazardous area which would require a special motor (Explosion Proof, Severe Duty)?
  2. Mentally locate the pump or system. Where is the injection point? Does the arrangement allow convenient access? Is the unit to be indoors or outdoors? What are the temperature extremes to which the unit will be subjected?
  3. Select the pump size-Determine flow rate, pressure and materials of construction. Are there special considerations for high viscosity or slurry?
  4. What method of control will be used? Manual continuous operation; on/off operation; or, proportional to some process signal.
  5. What container will hold the chemical? A new tank or an existing tank? A day tank or a bulk-shipping container?
  6. Locate pump near the feed tank. Keep the suction line short. Metering pumps will “push” against great pressures but they will not “pull” for very great distances. Limit the total length of the suction line to 3 feet suction lift or 6-7 feet flooded suction. Use an adequate sized line. Minimize bends, elbows, or other restrictions. Flooded suction is always preferred-easier to prime-more “forgiving”. Flooded suction must be used for fluids such as alcohol where the vapor pressure could be less than the suction lift. Be aware when piping the system, of the location of the valves, unions, flanges, compression fittings, etc. Are they easily accessible where they are located?
  7. Consider the accessories:
    1. Suction strainer-Always use a suction strainer, 40-60 mesh to prevent foreign matter from getting into the ball checks of the metering pump.
    2. Isolation valves-Provide on both suction and discharge for ease of maintenance. Select large port quick opening valves. A ball valve has a generous opening and is easily stroked from full close to full open position. A needle valve would not be an acceptable suction valve, as it would pose a restriction. Note, also it is not obvious when a gate or globe valve is fully opened or only partially opened.
    3. Calibration column-The pulsed flow of positive displacement metering pumps and the fact that metering pumps are often used in very low volume applications make a suction draw down column the most accurate and convenient method to measure pump performance.
    4. Relief valve-Does the pump have an internal relief valve? Will an external relief valve be required in addition to the internal relief valve?
    5. Back pressure valve-Required when a system does not provide backpressure and the pump does not contain a backpressure device. Note: A partially closed valve is not an acceptable backpressure regulator. A spring loaded, diaphragm type back pressure valve is required to provide proper back pressure Always use a back pressure valve when feeding from a bulk tank to an injection point with little or no back pressure-do not depend on spring loaded pump valves for this application. Backpressure valves are required when a low-pressure injection point is hydraulically lower than the feed tank. If a backpressure valve is not installed under these circumstances fluid can siphon and pump rate may be erratic, often pumping at a rate higher than the actual dial setting.
    6. Pressure gauge-Is a gauge required? Use a snubber on the gauge for pulsating services. A diaphragm seal must be used for chemicals which are corrosive to the stainless steel gauge parts or which are very thick or contain particles that could clog the Bourdon tube within the gauge.
    7. Pulsation dampener-Always discuss the requirements and goals of pulsation dampening with the manufacturer. Provide the reason for dampening and the degree of dampening required. Long pipe runs between the pump and injection point may need pulsation dampeners to reduce water hammer or pressure spikes caused by acceleration of the liquid in the discharge line.
  8. Selection and sizing the pipe and accessories:
    1. Suction piping-The single, safest rule of thumb for selecting suction pipe size is to use one size larger than pump suction connection. Piping may be the same size as suction connection for slow speed pumps used with low viscosity chemical. As a practical matter, do not use hard piping smaller than 1/2″. For low pressure, low temperature, low flow applications which use tubing, 3/8″ is a practical minimum size.
    2. Discharge piping-Take care to select or specify piping suitable for the discharge pressure. Discharge pipe size is not as critical as the suction pipe size. Matching the pipe size to the discharge connection size is sufficient. Keep in mind the practical minimum size of 1/2″ for pipe and 3/8″ for tubing.
    3. Calibration columns-Columns should be sized to allow at least a one-minute test. Lower capacity pumps may be better served by a two-minute test. Use a tall, thin column for ease and accuracy.
    4. Pressure gauge-Gauge should be sized 30% to 50% larger than maximum expected pressure. Caution-consider the relief valve pressure, not the operating pressure. Example: A 150-psi gauge would adequately serve a 100-psi injection service, however, a 100-psi application with a 150-psi relief valve would require a 250-psi gauge.
    5. Relief valves-Size at 50 psi or 10% above the normal operating pressure. Use whichever figure is greater. Normally relief valve return is piped back to the tank. Transparent return tubing is preferred over hard pipe so that when the relief valve is opened fluid can be observed in the line.
    6. Back pressure valves-Valves should be set to provide a minimum of 50 psi.
  9. Helpful hint: When replacing equipment, it is best to ask a few questions of yourself and of the existing installation. Will your program operate at the same feed rates as the previous program? Is the equipment properly sized for your products? How well has the equipment been operating? Any problems with reliability, accuracy, unusually high maintenance requirements? There is no better start to a new chemical feed program than to ensure that chemical is delivered accurately with trouble free equipment.

There is no more sure method to avoid problems with chemical feed equipment than to take the time to properly select the size and type of equipment from the beginning.

Metering Pumps Trouble-Shooting Guide

Some problems arise with greater frequency than others.

By understanding how and why, preventative measures can be applied to reduce productivity loss, maintenance costs, and operator frustration.

The ability of hydraulic diaphragm metering pumps to inject precise, controlled amounts of chemicals under pressure makes them particularly well- suited to a wide variety of process control applications. Keeping these pumps up and running efficiently demands careful attention to proper installation and maintenance procedures.

Fortunately, most metering pump difficulties are rather easy to correct, particularly since most manufacturers make it a point to test each pump before shipment to their customers. Instead, problems most often stem from other components in the pump installation — piping, for example, or control mechanisms — or with the way the pumps are installed.

Some metering pump-related problems seem to occur with greater frequency than others. By understanding how and why, preventive measures can be applied to reduce maintenance. What follows are a few of the more common problems pump users face.


One of the first decisions faced by pump users is where to locate the pump. This may seem like a very simple matter, but all too often its where many pump problems begin. An ill-considered placement, where the pump is exposed to extreme temperature conditions, or too far from the supply vessel, can be the source of considerable trouble down the road.

With this in mind, your first step should be to read the manufacturer’s instructions regarding installation, and follow the recommendations.

Be sure to mount your pump on a firm, level surface, and allow sufficient access for routine maintenance. Ideally, the suction connection should be below the supply vessel’s liquid level to allow full flooded suction. The pump also should be located as close

3. Clogged or blocked suction strainer.

4. System discharge pressure greater than pump internal relief valve setting.

5. Starved suction.

Pumps properly installed with piping fully supported prevents stress on component connections. The installation of unions will help simplify pump servicing to the supply vessel as practical, to help minimize friction loss in the suction piping.

Always take into account the environment in which your pump will be located, since extreme temperature fluctuations, particularly on pumps installed outdoors, can have a pronounced effect on metering pump performance. For example, pumps installed where temperatures fall below freezing should be equipped with a heat source to prevent chemical freezing.

It’s also important to change hydraulic oil in your pump to reflect changing temperature conditions. In addition, you’ll want to sufficiently protect all components from rain, snow, and ice. Failure to do so could result in a situation similar to the following:

–Clean or replace (suction line was not flushed prior to making connection to pump, permitting solids or debris such as pipe sealant, tape, etc. to enter and block check valves).

–Check and reset relief valve (within pump rating).

–Insufficient NPSH. Shorten suction piping, increase suction piping size or suction head.

Probable Cause

  1. Insufficient hydraulic oil.
  2. Clogged or blocked check valves, or check valves held open by solids.


Fill to proper level

Problem: A leading Gulf Coast chemical manufacturer experienced total operating failure shortly after start-up of several new metering pumps equipped with electronic capacity control actuators.

Solution: The service technician discovered that the installation contractor had removed the pumps! actuators from factory-supplied baseplates, resulting in serious misalignment problems.

Although installed outdoors, the contractor had wired the pumps and actuators using indoor-type non- watertight electrical connectors. This allowed rain to thoroughly penetrate wiring and enter the pumps and actuators, shorting-out critical electronic components.

Alter realigning pumps and actuators, rewiring them with watertight electrical connectors, and replacing the damaged electronic parts, the pumps operated properly


Nearly 85% of all metering pump operating problems can be directly attributed to suction difficulties, either because of undersized suction piping or due to blockage and!or restrictions in the suction line.

Unlike the steady flow characteristics of a centrifugal pump, a reciprocating metering pump with its pulsating flow requires piping large enough to handle the peak instantaneous flow, which is three times greater than the rated pump capacity. Thus, a metering pump rated at 60 gph produces a 188 gph peak instantaneous flow rate. (60 gph x 3.14 = 188 gph)

Problems can be avoided by keeping suction lines as short and as straight as possible. Piping should be sloped, if necessary, to eliminate vapor pockets. Although suction pipe size requirements vary greatly with each application, a good ‘rule of thumb” is

Probable Cause

  1. Partially clogged/dirty suction strainer
  2. Insufficient hydraulic oil
  3. Leak in suction piping
  4. Internal or external relief valve is relieving
  5. Insufficient suction pressure
  6. Worn or dirty check valves.
  7. Liquid close to boiling point
  8. Liquid viscosity too high


  1. Clean strainer
  2. Fill to proper level
  3. Repair piping
  4. Reset valve
  5. Raise liquid tank level
  6. clean or replace
  7. Cool liquid or increase suction head
  8. Reduce viscosity of liquid; change ball material and/or size.

Pump motor fails to start

Probable Cause

  1. Blown fuse or tripped breaker
  2. Open thermal overload in motor starter
  3. Low line current
  4. Open circuit in limit switches, timers, or other control devices in pump motor starter circuit,
  5. Motor damage


  1. Replace fuse after correcting cause of overload.
  2. Reset after correcting cause of overload. If malfunction recurs, check heater size.
  3. Determine cause and correct.
  4. Reset
  5. Check motor for physical damage that may hinder operation.

Pump runs but fails to deliver

Probable Cause

  1. Insufficient hydraulic oil
  2. Check valve(s) lodged open by solids.
  3. Worn or dirty check valves.
  4. Suction or discharge line blocked.
  5. Isolation valve closed.
  6. Pump is not primed,
  7. Check valve(s) installed incorrectly,
  8. Solids buildup between diaphragm and contour plate, preventing diaphragm movement,


  1. Fill to proper level.
  2. Clean or replace.
  3. Clean or replace.
  4. Clean line.
  5. Open valve.
  6. Allow suction line and pump head to fill with liquid before pumping against pressure.
  7. Remove and reinstall correctly.
  8. Remove and clean liquid end and replace diaphragm (infrequent occurence when pumping liquids containing solids that settle out. Tubular diaphragm liquid end highly recommended).

Pump fails to deliver rated capacity

Probable Cause

  1. Incorrect capacity setting.
  2. Insufficient hydraulic oil
  3. Starved suction
  4. Internal or external relief valve set too low for system conditions and is relieving.
  5. Leaky suction piping
  6. Excessive suction lift
  7. Liquid close to boiling point
  8. Liquid viscosity too high
  9. Worn or dirty check valves


  1. Re-adjust capacity setting
  2. Fill to proper level
  3. Increase suction piping size or suction head.
  4. Reset valve to correct setting and within pump rating.
  5. Repair piping
  6. Decrease lift
  7. Cool liquid or increase suction head.
  8. Reduce viscosity of liquid; change ball material and/or size.
  9. Clean or replace

Pump operates erratically

Never use pipe that is smaller than the suction connection furnished by the pump manufacturer. Also, support piping so it does not strain the pump connections.

A strainer with properly-sized screening should always be installed in the suction line to prevent debris or undissolved chemical particles from entering the pump and interfering with check valve operation.

Before connecting the pump, especially in new installations, flush the suction piping thoroughly. Failure to complete this important step could result in construction debris entering and damaging the pump, and possibly voiding the warranty in the process.

Problem: A customer reported an inability to obtain 100% capacity from several metering pumps installed in a suction-lift application. The manufacturer’s NPSH calculations indicated that the pumps should operate properly with the end user’s 3/4” Schedule 40 suction pipe.

Solution: The service technician asked the customer to provide the factory with a detailed sketch diagramming the suction piping arrangement—which did indeed include 3/4” Schedule 40 pipe However, the pump user’s drawing also revealed a 12” long flexible hose with integral quick-connect type fittings installed in the suction line.

Consulting the hose manufacturer, the service technician learned that the quick-connect fittings had only a 1/4” l.D., which resulted in a more than 80% reduction of the l.D. of the suction line. The customer was advised to replace the hose and quick-connects with larger components, which solved the problem.

Problem: A major chemical producer experienced pump failure immediately upon startup of a new production line at one of their plants. Operaters adjusted pump capacity controls, but could not get the new pumps to feed chemical, and called to request factory assistance.

Solution: The serviceman determined that the contractor had neglected to flush the piping system prior to installing pumps. Cigarette butts, dead bugs and metal chips left over from construction had accumulated in the supply tank and piping, and were flushed into the pump on startup, completely blocking check valves.

Plant operators were instructed to disconnect and flush out the piping and pump check valves, and install a properly sized suction strainer. The pumps operated properly after this was accomplished.

Pump delivers too much capacity

Probable Cause

  1. Insufficient discharge pressure
  2. Too much suction pressure


  1. Increase discharge pressure. (pump discharge pressure must be 15-30 psi greater than suction pressure to assure proper ball check valve operation).

Noisy operation of BaH check valves

  1. Excessive gear wear
  2. End play in worm (motor) shaft
  3. Improper lubrication
  4. Worm bearings

Ball valves make a clicking sound as they operate; rattling noises may also be heard under certain conditions these noises are amplified by the natural resonance of piping. These noises are normal and should not cause concern.

  1. Repair gear set
  2. Reshim shaft
  3. Replace with correct oil
  4. Replace bearings

Motor overheats

Probable Cause

  1. Motor overloaded due to operating in excess of pump discharge pressure rating.
  2. Improper wiring or low voltage
  3. Hydraulic oil too viscous
  4. High ambient temperature


  1. Limit discharge pressure to maximum specified.
  2. Check power supply vs. motor nameplate data.
  3. Drain and refill with correct hydraulic oil.
  4. Increase ventilation or relocate pump. If outdoors, provide sun shield.

Pump leaking oil

Probable Cause

  1. Oil plug loose
  2. Pump overfilled with oil
  3. Breather cap loose or missing
  4. Diaphragm head bolts not tightened


  1. Tighten
  2. Drain oil to proper level
  3. Tighten or replace
  4. Tighten to specifications

Pump losing oil or hydraulic oil discolored

Ruptured diaphragm

Replace diaphragm after correcting cause of failure. Pump hydraulic oil must be discarded and gearbox thoroughly flushed of all traces of process fluid. Replace with fresh hydraulic oil.

* Motors can safely operate at total skin temperatures between 95° C to 130° C, depending upon enclosure and insulation system. This results in normal “hot” temperatures which should not cause concern.


Sizing of discharge piping becomes more critical as pipe length increases. Long pipe runs create higher pressure drops and are subject to mass inertia effects, resulting in the hydraulic shocks commonly known as “water hammer”. In such applications a pulsation dampener, installed as close as practical to the pump discharge connection, will do much to improve pump performance.

Discharge pipe should be adequately sized for the application, and in a pressure rating in excess of the pump’s design. All valves and fittings should be full-ported to prevent restrictions that could affect pump performance.

Problem: An equipment fabricator supplied metering pumps installed on chemical injection skids. Upon start up, the pumps would not deliver 100% capacity and the pumps’ internal relief valves actuated continually at capacity setting above 50%. Repeated attempts by the equipment fabricator’s serviceman to solve the problem proved unsuccessful.

Solution: After concluding that the pumps were notihe problem—contrary to the end user’s adamant claim —the pump manufacturer suggested the end user disassemble the insulated downstream piping for inspection.

This led to the discovery that an undersized pipe adapter had been installed In the discharge line, thereby creating a restriction which limited flows to 50% of the pumps’ capacity. The end user was advised to remove and replace the restrictive fitting, which finally resolved the problem.

Problem: A cogeneration plant had installed several low-flow Diamyte® metering pumps.Cn start-up, operators became alarmed when, after several hours, no pumping could be observed at the injection point Assuming that the metering pumps were inoperable, operators shut them down and called for factory field service.

Solution: A technician visited the site, andt discovered that the contractor installed the pumps using 1” diameter discharge pipe, which ran a distance of more than 100 feet to the injection point.

Based on the pump’s n?aximum flow rate of 0.14 gph, the technician calculated that itwould take nearly 29 hours to completely fill the discharge line and before any “pumping” action would be observed at the injection point. The plant was advised to replace the 1” piping with 1 0.0. high-pressure mechanical tubing—the proper size for this particular application.

< Extreme temperature fluctuations can significantly impact the performance of metering pumps installed outdoors.


Problems can also occur with accessories such as back pressure and safety relief valves, pulsation dampeners, calibration cylinders, and controls (Figure 1). All must be accurately sized and installed, since improperly matched components can adversely affect pump performance.

Problem: A leading computer chip manufacturer requested warranty service from the pump manufacturer, claiming recently installed metering pumps with electronic controls were not delivering the proper flow rate, based on instrument readings taken from a newly installed PLC.

Solution: Visiting the jobsite, the service technician immediately determined that pump performance was precisely to specifications. He then checked the signal readings from the PLC and quickly determined that it was incorrectly calibrated, resulting in false signals to the pump’s electronic capacity control. After the PLC manufacturer recalibrated their unit, the pumps operated properly.

Loose or improperly installed pipe and fittings can allow process chemical to drip on the pump and ancillary equipment — to the detriment of system efficiency.


A measure of prevention is all that’s needed to help keep your pump operating at optimum efficiency.

>Read the instruction manual.

Easily done, yet frequently overlooked. Many problems can be avoided simply by reading and following the manufacturer’s instructions. These can provide a wealth of information not only on the installation of metering pumps, but also the proper start-up procedures necessary to avoid problems.

Establish a regular maintenance schedule. A little preventive maintenance can go a long way in keeping problems from developing. Check oil levels periodically, and schedule complete oil changes once a year.

>Keep spare parts on hand, and replace worn parts as necessary Most pump manufacturers offer packaged recommended spare parts kits, which include those parts you’ll most likely need to keep you pump up and running.

>When a problem arises, provide the manufacturer with complete details. The more the manufacturer knows, the faster they can help solve your problem. Tell all — everything from the pump’s location, capacity, and materials of construction, to the accessories involved and the chemical being pumped. It’s also a good idea to provide a sketch of the entire installation.

>Use a pump only for the application for which it was specified. If your application changes, be certain that the pump liquid and material of construction are compatible with the process fluid of the new application. If you have any questions or doubts, consult the manufacturer first.

>Above all, ask questions. If there’s something you don’t understand about your pump, ask the manufacturer. They can provide the information you may need to solve problems before they arise.

Improperly matched components can adversely affect pump performance. All must be accurately sized and installed.


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