Many processing operations require accurate dosing of liquid additives. Most often, plants rely on metering pumps to deliver the precise flows needed. Generally, such pumps feature a piston (reciprocating cylinder) somewhere within them.
In concept, metering pumps include three sections: 1) a drive mechanism —a device that allows the capacity to be adjusted during operation; 2) a prime mover — a power source to drive the pump; and 3) a liquid (or wetted) end — a mechanical configuration to deliver the pumped fluid.
Manufacturers of metering pumps usually supply a complete assembly that includes the metering pump, control elements, additive storage tank and other auxiliaries. When correctly specified, the vendor package will meet process requirements. So, carefully consider some key factors before purchasing a metering pump or making service changes and when troubleshooting.
First, check materials’ specifications. Do the pump materials of construction meet service requirements? Don’t forget to include seals, packing glands and other trim items in the review. Replacing trim items with more-suitable ones often can allow a pump to perform better in its current duties or handle a new service. However, larger items such as pistons and cylinders may cost enough that a new pump becomes a more-attractive option.
Second, check pressure, temperature and power ratings. Will the metering pump safely meet process requirements? Assess both the pump and the system for abnormal as well as normal operating conditions. Will the pump face problems if process upsets cause very high discharge pressures? Does the pump have the correct mechanical rating? Most additive tanks are at atmospheric pressure. Even if the mechanical equipment can handle higher pressures, can the driver deliver enough power?
Third, does the pump have sufficient operating range? Most metering pumps will work well for a range of 10–90% of nominal capacity. Operating at very low rates may require more-complex control systems. Often, this involves pairing two or more control techniques. For example, one method to get an accurate 5–90% range uses both a variable-speed motor that could turn down to 25% of capacity and a stroke adjustment (a slider crank) that could adjust the stroke length to 20% of capacity. This results in a minimum accurate rate of 0.25 × 0.20 or 0.05% of rated capacity.
Fourth, is flow control down to close-to-zero level needed? Variable-speed electric motors typically require some minimum speed for reliable operation. This may limit the lower capacity of the pump. Mechanical adjustment of the stroke length or stroke volume, or pneumatic speed control work much better at low flow rates. Closed-loop control systems measure a downstream point after the additive goes into the process to set additive demand. These systems are more likely to require turndown to very low or zero flow rates. Open-loop control systems measure an upstream point before the liquid addition to set its rate. Open-loop systems commonly handle corrosion-control additives whose dosing is set as a fraction of the flow rate. Such systems also tend to have more-consistent additive rates and are less likely to require very low additive rates.
Fifth, is the additive compatible with the liquid end? Plunger-type pumps always leak slightly. Some leakage results from wear but some is needed to keep the pump cool. Plunger pumps suffer from excessive wear with abrasive fluids. They also may present problems with toxic fluids. Diaphragm-type liquid-end pumps better meet the requirements for abrasive and toxic fluids.
Sixth, does the system have the necessary auxiliary equipment? Many additives require agitation to prevent solids from setting. Does the storage tank have an agitator? Others may freeze or gel at relatively high temperatures. Does the storage tank include the needed heating or insulation?
These are just some basic factors to consider for metering pump applications. (For more on such pumps, see: “Master Metering Pumps.") Properly understanding your equipment is the key to getting the most from your plant.
ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can email him at [email protected]