If you imagine a single plunger of the pump to move approximately  in accordance with a sinus, the plunger head location could be represented as x(time) = Half_stroke x sin (rotational_speed x time). The liquid in the suction line has to follow this plunger movement during the suction stroke. In the pressure stroke no liquid enters the pump at this plunger and is decelerated.

When the plunger is again in a suction stroke it pulls again on the suction line column back to the suction vessel. The pressure can locally not become lower than absolute vacuum. If the pressure locally becomes below the vapor pressure a vapor bubble will appear. The pulling force is limited by the vapor pressure. When the plunger is decelerating or in pressure stroke the liquid in the suction line catches up and will bump on the pump.

Differentiating the indicative equation of above, the acceleration at the head of the plunger becomes: a(t) = -  Half_stroke x rotational_speed ^ 2 x sin( rotational_speed x time). In order not to have a vapor (or vacuum) bubble will the liquid at the plunger (and the entire suction inline) with mass m have to follow this acceleration. The required force to accelerate this mass m is F =m x a. This force F divided by the cross section A of the suction pipe gives you an indication of the local pressure required to achieve the acceleration.

In order to reduce the risk of vapor (or vacuum) bubble creation at the suction of the pump:
?    The suction mass to accelerate is reduced by placing a pulsation dampener (only the suction line part up to the pulsation dampener has to follow the plunger head), and/or
?    The suction vessel pressure is increased.
 
The factor: Half_stroke x rotational_speed ^2 could be compared between both pumps in order to give you an indication of the difference in pulling and suction pressure. The above is only indicative, as might be clear to you.You are right, the bigger pump should cause more pulsations than the smaller as its stroke volume is bigger.

Anti-vibrations pads between pump base frame and steel construction or foundation are recommended as they reduce vibrations. For the same reason a suction side expansion joint (compensator) is a nice thing to add. But the vibrations do not affect the flow meter reading or the pressure reading on the gauge.

I think Frank is right with his suggestions although some of his assumptions might not be correct. I beleive your pumps are triplex plunger pumps, correct? If so, there is flow all the time into the pump because at any time at least one of the plungers performs a suction stroke. But the suction side of the bak-up pump is much smaller if it has the same flow as the bigger pump. Not only the suction inlet but also the valves are much smaller. That means the velocity inside the pump and the accelerations are huge. Most probably the back-up pump cavitates. This would explain the erratic flow meter readings on the discharge side, too.

What can you do? I think not much. You can enlarge the suction pipe of the back-up pump but this will only help if the existing suction pipe is rather long and small in size. But as the this will not change the velocity inside the pump (in the suction manifold and through the suction valves) the effect might only be small. What was the reason for the smaller back-up up pump (smaller in size, not in flow). Let me guess: price?
Sweage pumps &Sludge pumps

2011-05-27

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