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The Ins and Outs of the Inlet Manifold System of Your Mud Pum

Ron Lowe

The majority of problems occurring with reciprocating pumps originate on the suction side of the pump. The term “suction” can be misleading due to the fact that single acting pumps do not lift liquid or suck. A better term to use is “the line leading to the inlet manifold on the pump.” Single acting pumps rely upon atmospheric pressure to force the liquid into a low pressure area created by the completed discharge stroke of the piston or a centrifugal pump with sufficient head and flow to deliver liquid to the inlet manifold as needed.

The inlet side of the pump is critical to correct operation. Collapsible hose should never be used on the inlet side and length is always a factor because friction loss will enter the picture. The inlet line should have a large inside diameter, straight and short as possible. This type system yields a huge benefit to the cost of overall maintenance of the pump. The inlet to the pump should be as close to the supply source as possible and sized according to the discharge flow of the pump. Never decrease the inlet manifold diameter of the liquid end in order to use a smaller inlet hose. Always increase the inlet hose by 1 or 2 in. to ensure good inlet conditions for the pump. The inlet velocity should never be less than 1 ft per second or greater than 3 ft per second.

When problems start to occur on the inlet side, those problems can be both visual and audible. The inlet line will start to pulsate. This occurs because of the liquid accelerating and decelerating rapidly. On each rotation of a triplex crankshaft, one inlet valve will be closed and another will be partially open. The third will be fully open on the discharge stroke. This movement of the valves causes the liquid within the supply line to try and stop, start toward the pump and reverse toward the supply all at the same time. Therefore, poorly designed supply lines will show this movement in pulsating form.  

This pulsation can also be audible within the pump. You will hear hammering and if severe enough, the discharge line will also began to pulse. If this much cavitation is occurring within the pump, severe damage can and will occur within the power end as well as the liquid end if conditions are not changed. The hammering sound is the audible form of pump cavitation.

Cavitation is caused by insufficient flow or head from the supply tank to the inlet of the pump and the smooth transition from the manifold through the throat of the inlet valve. If the liquid is too viscous or the supply of the liquid is insufficient to meet the required speed (gpm) of the pump, gasses will start to break out of the liquid and form bubbles in the slurry as it passes through the throat of the inlet valve. When the pump is on the discharge stroke and pressure starts to build rapidly from atmosphere to discharge pressure, the bubbles start to implode and cause damage to liquid end components as well as the liquid end itself.

A suction valve or inlet valve is nothing more than an orifice within the system. The suction manifold of the pump should be filled with liquid and moving at a consistent flow rate. As an inlet valve begins to open and liquid flows through, the liquid should be in contact with the face of the piston. This column of liquid should move at the same rate as the piston. If the inlet is inadequate or pump speed too great, gas will break from the liquid in the low pressure zone created by the inlet seat and bubbles will from in the pumping chamber or atmosphere will enter through the liner and piston. Either way, the pumping chamber will not be filled with liquid.

When the discharge stroke begins, the piston is at rest. Half way through the discharge stroke, the piston is traveling at maximum velocity. Ideally, the pumping chamber should be filled with liquid and in contact with the face of the piston. The inlet valve within that chamber should have just closed as the piston starts the discharge stroke. If bubbles have formed, the chamber is only partially filled with liquid. As pressure starts to build within the chamber, the bubbles implode and the piston encounters a partially filled chamber traveling at maximum velocity slamming open the discharge valve and causing a shock wave to be sent throughout the entire system.

If a pump has a satisfactory inlet system, the end user will see the results of that in his hip pocket. If not, he will help support the entire industry with more equipment sales.
on Lowe is regional manager with Myers Aplex, a Pentair Industrial company
Courtesy trenchless Technology

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