Losing system pressure - positive displacement pumps

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Courtesy of Cat Pumps

In a positive displacement pump, flow is directly related to RPM and pressure by the restriction of the flow in the discharge line. Once the system pressure is set, any change in flow will result in a change in pressure. These changes in flow can occur from certain conditions in the pump inlet line, the pump itself, the pump drive or in the pump discharge line.

Pump Inlet

The inlet conditions are often one of the first places to investigate when a system loses pressure. Many things can cause cavitation or starvation and result in a gradual loss or fluctuation of system pressure.

When setting up a system, it is critical, to achieve optimum performance, that the inlet conditions are adequate to handle the amount of flow, the type of liquid and the temperature. Excessive heat, abrasives and low-lubricity liquids require special attention to the inlet supply line and can accelerate the wear of pump components and system accessories resulting in reduced system pressure.

The best supply situation for a positive displacement pump is a positive feed with a baffled supply tank sized 6-10 times the system capacity; a reasonable line length to the pump of under 6 feet; a line size equal to or one size greater than the pump inlet; a minimum of 90° elbows and flexible hose to the pump to reduce pulsation and surges in flow.

If these ideal conditions are not possible, and longer feed lines and booster pumps are necessary, additional protection devices, such as a captive acceleration tube [C.A.T.], are recommended to maintain consistent inlet pressure.

Of course, it is recommended that other precautions be taken at the inlet of your positive displacement pump to minimize problems. All liquids should be filtered before entering the pump to minimize wear to the pump V-Packings [seals] and valves. A flow control valve should be installed in the tank to maintain adequate liquid levels to avoid starving the pump. A temperature control valve should be installed to maintain liquid temperature within the pump limits. Overheating will quickly deteriorate both V-Packings [seals] and valve assemblies and reduce system pressure.

Problems in other inlet accessories can also contribute to losing system pressure. Clogged filters are a prime target. These should be inspected and cleaned regularly. The supply, tank if not properly baffled, can permit excessive agitated liquid that contains air bubbles to enter the pump. This can result in cavitation and erratic or diminishing pressure. The tank should have at least two baffles, secured to the tank bottom, to allow the liquid to flow over and expel unwanted air before entering the pump feed line. All fittings and hoses should be periodically inspected for air leaks as this will also contribute to a loss in system pressure.

Pump Drive

Changes in the RPM of your drive, may contribute to a loss of system pressure.

A proper and secure drive for the pump is a significant factor in maintaining a consistent flow and the resulting system pressure. When initially setting up the system, carefully calculate the HP required using the flow and pressure and efficiency of the pump. Then review the pump RPM and drive RPM and select the proper size pulleys to achieve the desired flow. Review the maximum horsepower per belt to assure that the pump is receiving adequate power to deliver the desired flow. Establishing the correct belt length and center distance is necessary for the proper HP. If in doubt, consult your pump and/or drive supplier for their recommendations. Replace belts on a regular schedule to maintain maximum HP. Worn or slipping belts, the wrong pulley sizes for the pump and/or motor and an undersized drive can alter the desired pump output and directly affect the system pressure.

At The Pump

Changes within the pump can also affect system pressure. Good pump maintenance is an essential element in maintaining consistent flow and system pressure. Regular servicing of the V-Packings [seals] and valves will assure good sealing and seating surfaces within the pump and optimum performance. Pump maintenance should be established specifically for each system as liquid, temperature, operating cycles and system accessories all affect the wear on a system. With a pressure gauge at the pump, it is easy to see when the system pressure drops or fluctuates. This is the time to do maintenance. Do not wait until the pump leaks externally or quits running completely to do maintenance. Excessive and expensive damage may occur.

Discharge Line

This is a primary place to investigate when the system begins to lose pressure. Most often, worn nozzles are at the source of a system pressure loss and too often adjustments are made in the regulating and relief valves to compensate for this nozzle wear. This can put unnecessary stress on the pump. If there is no pressure gauge in the system, you may exceed the pressure limit of the pump when making adjustments. Always replace nozzles as a first step to correct a pressure loss. If the nozzle does not restore the system pressure to the original setting, then proceed to other inlet, pump or discharge conditions.

Pressure regulators or unloaders can also be the cause of pressure losses. Internal seats, pistons and o-rings can wear and reduce the ability of the device to hold set system pressure. Worn check valves can cause a device to cycle and give erratic pressure readings. It is important to inspect system accessories at the same time pump maintenance is performed.

It is best to establish a complete maintenance routine for the entire system that includes the pump, filters, hoses, connections, valves and nozzles. Regular maintenance is far less expensive and time consuming than major overhauls or replacement. Good maintenance of your positive displacement pump system will assure consistent flow and pressure.

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