Common System Start-Up Problems
Most 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.
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)?
- 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?
- Select the pump size-Determine flow rate, pressure and materials of construction. Are there special considerations for high viscosity or slurry?
- What method of control will be used? Manual continuous operation; on/off operation; or, proportional to some process signal.
- What container will hold the chemical? A new tank or an existing tank? A day tank or a bulk-shipping container?
- 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?
Consider the accessories.
Suction Strainer – Always use a suction strainer, 40-60 mesh to prevent foreign matter from getting into the ball checks of the metering pump.
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.
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.
Relief Valve – Does the pump have an internal relief valve? Will an external relief valve be required in addition to the internal relief valve?
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.
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.
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.
Selection and sizing the pipe and accessories.
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.
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.
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.
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.
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.
Back Pressure Valves – Valves should be set to provide a minimum of 50 psi.
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.