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).
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.
Mechanically Actuated Diaphragm Design
A mechanically-actuated diaphragm pump represents the best balance between low pump cost and high quality performance. Because it has zero diaphragm leakage, it makes a great pump for critical and otherwise expensive chemicals or where environmental issues are involved.
The mechanically-actuated series is an excellent choice where slurries and abrasive chemicals are required up to the pump's maximum flow and pressure ranges. They are also well tolerant of high viscosity liquids, providing an economical solution for a variety of difficult applications.
Mechanically-actuated pumps operate with a plunger directly attached to the diaphragm. This attachment generally takes place from a bolt and clamp being placed through the plunger and through the diaphragm. The direct attachment of the piston to the diaphragm connects the pump's drive and motor to the liquid end. The motion of the pump drive moves the plunger back and forth, thereby causing suction from the supply tank and pumping the fluid of choice through the attached conveyance infrastructure.
This type of pump generally finds pressure peaks at 175-psi, but is only limited to flow as a matter of wetted end volume. Maximum life of the pump can be achieved by replacing the diaphragm at the recommended service interval. Leak detection can be easily found from the air-filled chamber residing generally at atmospheric pressure on the drive side of the liquid end.
As with any chemical where gas binding can be a problem, it is recommended that a degassing valve be used to release off-gases from the agitation or pressure changes experienced by a liquid having off-gas characteristics. Some of these liquids that can generate off-gases as a result of pressure losses are NaOCl, H2O2, and some specialty chemicals.
Mechanically-actuated pumps work well in these applications, providing 10:1 turndown as a standard. The addition of VFD technology and remote stroke control will bring the turndown as high as 100:1. Mechanically-actuated diaphragm pumps are easily maintained and provide years of service for little effort.