Drill Hole in Pump Check Valves 1

[brochon]

I would like to survey you folks to see:

How common is the practise of drilling holes in pump check valves (e.g. to act as warm up line on a spare pump)?

I have had this discussion many times in house and there are good arguments for and against. I would like to know just how common is the practise. If you have any strong arguments (one way or the other) please let me know.

Thx Brian

 

[PUMPDESIGNER]

On some of our systems a hole in that check valve would be incorrect. The reason is that the hole would result in applying pressure through that pump into the PVC intake line for that pump.

The PVC intake line would thereby be pressure cycled, which is the largest single cause of PVC fitting failures industry wide.

Of course on other systems we manufacture we have either steel, stainless steel, or HDPE intake lines, which would not be damaged by pressure cycling.

PUMPDESIGNER

 

[stewbaby]

Really a plant preference, but we install a 3/4" or so (depends on system) bypass around the check and install a minimum flow orifice plate or nipple (restriction orifice, RO). Include a gate valve for isolation of pump purposes. The drilled hole in the check flapper caused too many problems when troubleshooting pumping problems:

*Does the check have a hole? (impossible to verify warm-up is on going or why if spare pump got plugged / cold).
*Is it to big (too much flow coming back through)?
*if pump block valve starts leaking thru, then check allows product back to pump and prevents maintenance activities.

Both work, just easier to know what you have and control with a RO. Best wishes.

 

[vanstoja]

Check valve bybass orifices are commonly used in pressurized, high temperature fluid systems where multiple pump branches are employed and check valves are used to prevent or limit reverse flow into idled (standby pump?) branches. Pressurized thermal shock damage to piping or components is the concern if the fluid temperature in the idled branch departs too much from that in the operating pump branches (a 50 degree F differential may be a maximum allowed in pressurized water nuclear power coolant systems). I would not expect check valve bypasses to be necessary or critical for non-pressurized, low temperature, non-paralled fluid pumping systems. A bonus of temperature equalizing check valve bypasses in parallel pumping systems is some mitigation of the magnitude of check valve slam pressure surges in the system because the bypass orifice "bleeds off" the pressure as the check valve disk impacts the seat at high closing velocities. In sizing check valve seat orificies for fluid temperature equalization and/or pressure surge relief, the maximum flow allowed through the bypass should not be high enough to "breakaway" an idled pump and cause it to rotate in reverse direction under reverse flow conditions. A check valve seat bypass orifice establishes a maximum system resistance flow condition above shutoff (zero) flow which may or may not be sufficient to limit fluid temperature rise during startup of an idled pump with its check valve temporarily closed by the head of the running pumps. This may add a third consideration to check valve bypass orifice sizing besides fluid temperature equalization and pressure surge suppression.

 

[abeltio]

some systems require the orifice to maintain the std-by pump primed (e.g. on-base lube oil systems of gas turbines).
at all times.
HTH

saludos.
a.

 

[dave2109]

In my experience, the requirement to keep a stand-by pump warm depends on the design of the pump. Therefor, I'd stick to whatever the pump manufacturer's recommendations are. I've seen pumps that handle 375 deg F water that do not need any type of warm-up prior to start-up.
If you have a pump that does require warm-up, I prefer the orifice arrangement mentioned above by stewbaby as opposed to drilling a hole in the check valve since the backflow can regulated much better. Also, if you are dealing with high pressures, a multi-stage orifice can be provided to prevent flashing.

Pumpdesigners' comments above show the importance of providing a relief valve on the suction side of a pump when the suction piping is not rated for the discharge pressure. Even check valves without holes drilled in them tend to leak a little.

Another case where I've seen holes drilled in check valves is in colder climates where if the pumps are not running you want all the water to drain out of the system back through the pumps and into a freeze protected receiver.

 

[pablo02]

In line with what dave2109 stated, we use a small orifice in check valves in some limited applications to keep the water in the pipeline flowing (slightly) for freeze protection...