The following list will serve as a chemical feed installation and component checklist for municipal water and wastewater workers. Reviewing this list before meeting with metering pump vendors will help ensure that all requirements are addressed.
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 the suction connection for slow speed pumps used with low viscosity chemical. But hard piping smaller than 1/2" should be avoided. For low pressure, low temperature, low flow applications that use plastic tubing, 3/8" is a practical minimum size.
Discharge Piping— 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 usually sufficient.
Suction Strainer— Always use a suction strainer (40-60 mesh) to prevent foreign matter from getting into the ball checks of the metering pump.
Flanges, Unions or Compress- ion Fittings — These should be installed at the pump suction and discharge to facilitate maintenance.
Isolation Valves — Provide valves 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 closed to full open position. A needle valve would not be an acceptable suction valve, as the port design causes restriction.
Calibration Column — The pulsed flow of 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. 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 of reading and accuracy. A calibration column may be helpful in observing wear or dirt in the pump check valves. The liquid in the calibration column should draw down smoothly and stop smartly at the end of each suction stroke. If the liquid in the column "bounces", it would indicate the valves are worn or dirty. A calibration column may facilitate priming in a top mount installation.
Relief Valve — An external relief valve is recommended even if the pump has an internal relief valve. Set the relief valve at 50 psi or 10 percent (whichever is greater) above the maximum operating pressure. The relief valve return is piped back to the tank. Transparent return tubing allows fluid to be observed in the line if the relief valve opens. When portable, replaceable containers are used as the chemical source, it is convenient to pipe the relief valve back to the suction line. When piping the relief valve return to the suction, be certain the return is upstream of the pump suction isolation valve so that the flow path back to the tank cannot be blocked.
Back Pressure Valve — Required when a system does not provide adequate back pressure and pump does not contain a back pressure device. Back pressure valves are required when a low-pressure injection point is hydraulically lower than the feed tank.
It is important to note that a partially closed valve is not an acceptable back pressure 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. If a back pressure valve is not installed under these circumstances, fluid can siphon and pump rate may be erratic, often pumping at a rate higher than the dial setting. The valve should be set to provide a 50 psi minimum differential between suction and discharge.
Pressure Gauge — If a gauge is desired, use a snubber for pulsating services. A diaphragm seal must be used for chemicals that are corrosive to the stainless steel gauge parts, or that are viscous or contain particles that could clog the Bourdon tube within the gauge. Gauge should be sized 30 percent to 50 percent larger than maximum expected pressure.
It is critical to consider the relief valve pressure, not the operating pressure. For example, a 100 psi injection service would be served adequately by a 150 psi gauge. However, a 100 psi application with a 150 psi relief valve would require a 200 psi gauge.
Pulsation Dampener — Always discuss the requirements and goals of pulsation dampening with the pump manufacturer. Provide the reason for dampening and the degree of dampening required. The acceleration in a long discharge line can challenge the pump maximum pressure capacity or, at the very least, the relief valve setting. Use of a pulsation dampener will minimize the spikes caused by acceleration and, in the case of higher volume pumps, reduce piping harmonics.
Injection Quill and Check Valve — Install a quill at the injection point both to serve as a check valve and to provide better dispersion. Low-pressure applications may be better served by an injection quill with corporation stop.
