A Primer on Metering Pumps


Liquid End Designs

The liquid end, which is referred to as the wetted part of the pump, is selected to meet the specific service conditions of the application. Required flow and pressure ratings are considered, as well as the physical and chemical properties of the liquid. The liquid end's ability to protect the environment is also a major consideration when dealing with toxic or hazardous chemicals.
 All liquid ends have several features in common. First, the liquid is drawn into the wetted end by the rearward motion of a piston, and expelled by the forward motion. To achieve this, the metering pump is supplied with check valves at the suction and discharge connection points. The check valves contain and release the chemical based on system conditions and gravity.
During the suction portion of the stroke, the motion of the piston lifts the suction ball check from its seat, allowing liquid into the pump. At the same time, the piston's motion and system back pressure hold the upper check valve (discharge) closed. This is then reversed during the discharge stroke.
Check valves are available in several different designs and configurations. There are single or double ball configurations and poppet style check valves. Selecting which type to use can be determined by the manufacturer based on capacity required of the specific pump.
 For example, slurries or liquids with large fibers or particles can cause a single ball to leak if particles are trapped between the ball and seat. Therefore, a double ball check offers more stability and accuracy. On the other hand, since each check valve causes some resistance in the flow path even when open, viscous fluids are better handled with a single ball suction check valve.

Packed Plunger

The packed plunger style liquid end is the only liquid end in which the piston is in direct contact with the process fluid. This direct contact offers a number of advantages, including: high suction and discharge pressure capabilities; high temperature resistance, and lowest NPSH requirements.
The reciprocating piston requires packing to seal the wetted parts from the atmosphere. This simple design is effective, but places limitations on the use of packed plunger pumps in certain applications. Because a small amount of controlled leakage past the packing must be expected, this style liquid end should not be used with hazardous or toxic chemicals.
Additionally, the friction between the piston and the packing results in wear that increases leakage. Periodic packing adjustment is necessary to maintain volumetric efficiency. To avoid problems associated with leakage, consider a diaphragm style liquid end. The packed plunger can handle pressures up to 15,000-psi, and temperatures to 600-deg F (with special modifications).

Disc Diaphragm

Certain disc diaphragm liquid ends use a teflon diaphragm to act as a barrier between the piston and the process fluid. The piston's pumping motion is applied to hydraulic fluid which causes the diaphragm to flex back and forth as the piston reciprocates.
The hydraulically actuated diaphragm operates with equal pressure between the hydraulic and process fluids. This eliminates diaphragm stress, since the pressure is essentially equal on both sides at all times. Two contour plates encase the diaphragm to contain its travel.
The hydraulic and process fluids pass through carefully engineered holes in the contour plates in order to come into contact with the diaphragm. Relief and refill valves control the volume of hydraulic fluid. An automatic air bleed valve continuously purges air from the hydraulic fluid.
The diaphragm style pump is sealed, making it an excellent choice for hazardous, toxic, or corrosive chemicals. For extra protection, double diaphragm and leak detection modifications are available, although they are considered redundant since this design is extremely durable.
Because the process fluid must pass through relatively small holes in the contour plate, the disc diaphragm liquid end is not the best choice for slurries. With a few exceptions, disc diaphragms are usually not the best choice when pumping viscous fluids. The disc diaphragm is capable of handling fluids where the required injection pressure is 3500-psi or greater and the fluid temperature exceeds 250-deg F.

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