COMMONLY ASKED PUMP QUESTIONS

If a pump has low head pressure, a lot of times it’s because it isn’t running a valve that matches the head pressure, which will cause a lot of problems. If the valve is undersized it might not be capable of allowing a high enough flow rate. If the valve is oversized the system will be very sensitive. A “small” change for the oversized valve would create a relatively large change for the system.

• Low volumetric efficiency, or the failure to deliver rated capacity and pressure, can be caused by any number of conditions. Common sources include:
• Pockets of air or vapor in the inlet line
• A power pump capacity that’s greater than the charge pump capacity
• Air or vapor trapped above the inlet manifold
• Air leaks in the liquid supply piping system
• Air or gas entrained in liquid
• Foreign objects preventing the pump inlet or discharge valve(s) from closing or blocking liquid passage
• Incorrect drive ratio or motor/engine speed
• Loose belts, valve covers, cylinder heads, or bolts in the pump inlet manifold
• Worn valves, seats, liners, piston rings, or plungers
• A safety relief valve that’s partially held open or failing to maintain pressure
• A bypass valve that’s partially held open or failing to maintain pressure
• Blown liner gasket
• Insufficient NPIPA (net positive inlet pressure available)
• Internal liquid bypass
• Vortex in the supply tank
• Insufficient power delivery from the motor

Common cases of low NPIPA include:

• Partially clogged inlet lines
• Excessive liquid vapor pressure and pumping temperature
• Restricted inlet pipe fittings
• Too long of an inlet line
• Too many pipe fittings
• Too small of an inlet line
• Too low of static inlet head
• Too low of atmospheric pressure

The main drivers of pump overloads include:

• Excessive pump speed
• Low voltage or other electrical problems
• Trouble with engine, turbine, gears, or other related equipment
• Excessive discharge line pressure
• Clogged discharge lines or closed/throttled valves in the discharge line
• Incorrectly sized liner for the application
• Improper bypass conditions
• Over-tightened stuffing box glands on adjustable packaging

Stuffing box leakage primarily stems from the wrong size packaging or worn out parts including:

• Packing
• Rods or plungers
• Stuffing boxes
• O-ring seals

The three leading causes of stud failure are:

• Excessive discharge pressure
• Improper torqueing of nuts
• Shock overload from pump cavitation

Common causes of noisy valves include:

• Weak or broken valve springs
• Pump cavitation
• Air leaks in the inlet piping or loose bolts in pump inlet manifold
• Air trapped above the inlet valve

Common causes of inlet or discharge line vibration include:

• Inadequate piping support
• An inlet line that’s too long or small in diameter
• Too many bends in the inlet line
• Multiple pump installations operating in phase
• Obstruction under valve(s)
• Worn out packing
• Operating above the recommended pressure or speed
• Low NPIPA

Noisy operation can include both liquid and mechanical knocking, though the latter is much rarer on new installations. Causes of excessive noise include:

• Loose pistons or plungers
• Amplified valve noise through a power end
• Pump cavitation
• Air leaking into the inlet pipe
• Loose bolts in the pump inlet manifold
• Hydraulic noise in liquid end
• Loose or worn crosshead pins and bushings
• Loose connecting rod cap bolts
• Worn crosshead or connecting rod bearings
• Excessive main bearing end play
• Worn gears or chains
• Gears or chains that are out of line
• Backwards running pump
• Partial loss of prime
• Shocks in the piping system
• Water in the power end crankcase
• Poorly supported piping, abrupt turns in piping, misaligned piping, or piping that’s too small

In addition to normal wear over time, packing failure is caused by:

• Improper materials or lubrication
• Excessive gland tightening in adjustable packing
• Dirty liquid or environment
• Plunger or piston misalignment

In addition to normal wear over time, valve failure is caused by:

• Pump cavitation
• Abrasives in the fluid
• Incompatibility of valve components with corrosive liquids
• Electrolysis
• Improper installation including driving on the valve stem, improper torque on the jam nut, or valve seat/deck that isn’t thoroughly cleaned and dried

Common causes of plunger failure include:

• Thermal shock from cold water hitting the plunger
• Packing that’s too tight
• An inlet valve that becomes disassembled while the pump is in operation
• Improper tightening causing the stuffing box gland to rub on the plunger
• Dirty liquid or environment

• At times, liquid won’t be delivered because:
• The pump is not primed
• Air or vapor pockets are in the inlet line
• There’s a clogged inlet line
• Inlet valves or discharge valves are held open
• There are loose bolts in pump inlet manifold
• Valve velocities are too high

Cavitation often results from:

• Too low NPIHA (Net Pressure Inlet Head Available)
• Liquid not delivered to pump inlet connection
• Excessive stuffing box leakage
• Too high NPIHR (Net Pressure Inlet Head Required)

Leaks at the cylinder head or valve cover are caused by:

• Running above the recommended pressure
• Loose cylinder head or valve cover
• Damaged gasket

• The causes of water leaking into the crankcase include:
• Water condensation
• Worn out seals or crankcase packing
• Clogged air breather(s)
• Loose covers

Oil leaks from the crankcase are caused by:

• Oil levels that are too high
• Worn out seals or crankcase packing
• Loose crankcase cover

The power end can overheat as a result of:

• A pump running backward too slowly
• Insufficient or excessive oil in the power end
• Incorrect oil viscosity
• Operating above the recommended pressure
• Bearings that are too tight
• Belts that are too tight
• One or more of the cylinder’s discharge valves being stuck open
• Insufficient cooling or ventilation
• Low pump speed
• Excessive liquid end packing tightening on adjustable packaging