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Tuesday, March 23, 2010

Hydraulic Driven Diaphragm Pumps

Hydraulically driven diaphragm pumps are used in applications for the transfer or injection of chemicals into process streams at pressures up to 7500 lb/in2 (approximately 500 bar). Because the diaphragm is pressure balanced, the stresses in the diaphragm are low. Therefore, these pumps tend to require minimal maintenance. The pump’s capacities can be adjusted to match the specific process requirement by adjusting the effective stroke length or stroking speed of the pump. Effective stroke lengths are adjusted by either a hydraulic lost motion, a mechanical lost motion, or by varying the eccentric’s offset. The repeatability of the injected flow is plus or minus 1% or better.

Applications range from 0.26 to 26,000 gallon per hour (1 to 100,000 litters per hours). At flow about 26 gallons per hour (100 litters per hour), most pump models employ capacity adjustments based on variable eccentric or variable speed technology to avoid significant pressure spikes due to the rapid acceleration and deceleration of the fluid in the pipes.

As with the mechanical diaphragm pumps, a wide range of chemical can be handled. Wetted materials include PVC, Polypropylene, PVDF, 316 SS, Alloy 20, Alloy C-22, Titanium, and Inconel. Diaphragm for pressure up to 4350 lb/in2 (300 bar) are typically 316 SS, Alloy C or PEEK. Optional features include fluid temperature control jackets, diaphragm rupture detection capabilities, and remote diaphragm head design. Typical applications include the injection of acids and based for pH control, corrosion inhibitors, methanol, coagulants, primary process blending, process slurries, and drag reducers. These type of liquid ends are used, the disc diaphragm.

The disc diaphragm pump is equipped with process side and suction side restraining plates to prevent over displacement of the diaphragm during system upsets. When diaphragm reaches the suction the suction side restraining plate, the hydraulic oil pressure drops, causing the refill valve to open and replenish the oil. When the diaphragm hits the process side restraining plate, the hydraulic pressure rises, causing the relief valve to open, venting some oil. The fluid volume between the restraining plate is typically 150% of the maximum displaced volume of the pump. Therefore, the diaphragm does not contact both restraining plates during the same stroke.

The tubular diaphragm configuration is a variation of the disc diaphragm design. A diaphragm shaped in the form of a tube is placed in a chamber in front of the disc diaphragm assembly. This design eliminates the process fluid flowing through the front restraining plate, reducing viscose losses and wear in case of slurries. The chamber must be fitted with a precise amount of hydraulic fluid to avoid over displacing the tube.

The high performance diaphragm configuration eliminates the use of a process side restraining plate providing the through flow performance of a tubular design while eliminating the possibility of over displacing the tube during start-up and maintenance. With a mechanically arming, pressure sensitive refill valve, the hydraulic fluid can only be replenished when the diaphragm is in the most rearward position. This eliminates the possibility of overfilling the hydraulic chamber and therefore over displacing the diaphragm during system upsets (blocking suction or discharge lines).

Most problem with hydraulic diaphragm pumps occur due to incorrect system design. Pressure above 9 lb/in2 (0.6 bar) should be maintained in the pump diaphragm head during the suction stroke to stop vapour buildups in the hydraulic or process side cavities and special hydraulic fluids. NPSH calculation should include viscose losses in the check valves and contour plates (if so equipped).