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).

Pump for Chemical and Water Treatment Industries

Another type of mechanically driven diaphragm pump is used for the injection or transfer of chemicals into process streams at pressures up to 250 lb/in2 (17 bar). These pumps are designed to enable typically capacities can be adjusted through a 20:1 range. Injection capability is generally plus or minus 3%.

A wide range of chemicals can be handled. Wetted materials include PTFE or PTFE with elastomeric backing. Ball type check valves are usually employed.

Applications for this type of pump include the injection of acids and bases for pH control, biocides, chlorination, coagulations, and fertilizers. There are two basic configurations for pumps in this class; electromagnetic pumps, (solenoid) and motor driven pumps.

Electromagnetic (electronic) pumps are used in a variety of low power applications with flows from 0,026 to 26 gallons per hours (0.1 to 100 liters per hour) at pressures up to 250 lb/in2 (17 bar). These metering pumps employ an electronic control circuit that pulses on electromagnet that, in turn, generates the linear motion of an armature shaft diaphragm assembly. Each electronic pulse results in one discharge stroke of the pump. At the end of the stroke, a set of springs returns the diaphragm assembly to its initial position, drawing more fluid into the pump chamber in preparation for the next stroke.

These pumps are inherently safe, as they can be run indefinitely in the stalled condition without damage to the pump or over pressuring most systems. An additional feature of certain electronic pumps is the regulation of pulse strength through electronic power control, which leads to smoother fluid injection. Capacity is usually controlled by adjusting the stroke rate, but the stroke length can also be adjusted. Combining these adjustments provides a wide range of outputs.

Advances in the electronic controls have led to the capacity to control output manually from 4-20 mA process signals, digital pulses from external sources (such as flow meters), or serial data communications signals from computers. Yearly maintenance is recommended for low-pressure applications, but as pressure rise, diaphragm and check valves will need for low pressure applications, but as pressure rise, diaphragm and check valve will need to be replaced more frequently.

Mechanically Driven Diaphragm Pumps

Many industries are served by mechanically driven diaphragms pumps. They are used in construction, chemical, and water treatment applications.

Construction Industry

Mechanically driven diaphragm pumps are widely used in the construction industry for dewatering applications where pumps may ingest rocks or other debris. A popular make of this type of pump contains a spring on the plunger rod. If the operating pressure exceeds the maximum recommended pumping pressure, the spring compresses and does not more the diaphragm. The spring can compress and thus keep a rock from being pushed through the wall of the pumping chamber or cause the drive mechanism to fall.

In single diaphragm pumps, the pumped liquid can have a lot of inertia if the suction and diaphragm lines are relatively long. A simple accumulator on the suction (inlet) side of the pump enable the pump to draw liquid from the accumulator while it simultaneously draws liquid through the suction line.

During the discharge stroke, the accumulator can refill with liquid from the suction line. If the discharge line from the pump is relatively long, the inertia of the liquid can be great, as mentioned earlier, and can impose severe loads on the diaphragm and drive mechanism. The spring on the plunger and can absorb some of the drive energy early in the discharge stroke and “give it back” during the latter part of the discharge stroke, greatly reducing the inertia loading on the diaphragm and drive mechanism.

Mechanically driven diaphragm pumps in the construction industry operator by a reciprocating plunger, usually secured to plates on both sides of the diaphragm. The diaphragms are customarily fabric reinforced elastomers (usually synthetic rubbers) similar in many ways to the fabric reinforced materials used in pneumatic tires. The diaphragms are normally molded with a convoluted section between the central damaged area and the clamped periphery. This convoluted section permits longer strokes than would be possible otherwise.

These pumps are sometimes duplexed so that the reciprocating means acts alternatively on two diaphragms with one on a section stroke, while the other in on a discharge stroke and vice versa. A connector called a walking bean is pivoted between two diaphragms. As one diaphragm is pushed down on a discharge stroke, the other diaphragm is simultaneously pulled up on a suction stroke. The pumping chambers with inlet and outlet check valve are manifolded together to a common inlet and a common outlet. The principle advantage of the duplex diaphragm pump is its more constant flow (two pressure pulsa times per cycle).