For a properly designed system, relief from a centrifugal pump is caused by the pump circuit being blocked-in. The pump head rises along the curve. The actual pressure at the pump discharge will be the pump head plus the static suction head plus the source pressure. The pressure at the PSV will be this pump discharge pressure minus any discharge static head. There is no frictional loss since there is no fluid movement.

Assuming the suction static pressure and source pressure are constant at this time, there will be a point on the curve where the pressure in the system causes the relief valve to open. It could be anywhere on the curve or at complete shut-off; strictly a function of the pump curve. Once the PSV opens, flow begins with a bang! The pressure will fall along the curve and flow will increase. But so will frictional losses since we are now flowing. The pressure at the PSV may very well reach a point that will fall below the PSV blowdown, the PSV closes and flow stops. The cycle repeats itself.

The maximum flow through the PSV needs to be found using basic hydraulics, balancing the pump curve with system losses. This is the exact same calculation used to size the pump in the first place except now you have the pump curve to work with. There is no way to know, without calculating it, whether the maximum relieving rate is equal to, less than or greater than the design capacity of the pump. Assuming just the pump design capacity could be a big mistake or, it could be conservative. Only the true hydraulics of the system knows for sure. Magnetic pumps

2011-07-11

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