Rules on Juxtaposition of Valves

[alamin]

Hi,

I would like to know the Rule of Thumbs (DO's and DON'Ts) of installing multiple valves (same types as well as different types) side by side (in horizontal/vertical orientation) on a pipeline.

Today I came across an installation where a PTFE-lined ball check valve had been installed downstream of another rubber flap (with S/S mesh embedded inside to give strength) swing check valve as a second defence to failure of primary check valve. I thought the swinging of rubber flap would be restricted by the downstream ball check valve. This valving arrangement failed recently. Speculation is that scale build-up on flap and/or ball and seat might have caused the valving arrangement to fail. If there is no scale formation, can vacuum be created by this type of arrangement preventing full closure of ball check valve? The flowing media was sulphuric acid. There are in vertical orientation on the discharge line of a sump pump. Due to operational reason, it could be a while before these valves are removed for inspection.

Look forward to hearing your insightful thoughts.

Regards,
Amin

 

[zdas04]

I don't know a direct answer to your question, but I was once called upon to evaluate a measurement station that was apparently measuring 15% low. When I got there the first thing I saw was that they had a check valve on both the inlet piping and on the outlet piping (if one is good, two must be better, right?). With a stethescope I could hear them each banging on a pretty short cycle. Looking at the transducer output you could see the two check valves fighting each other. The pressure waves from the banging were on the order of 50 inH2O swings. When the the computer processed this data the result was low measurement. Due to convenience of the block valves, we pulled the flapper from the downstream valve and the station came into 0.5% of the sum of the check meters.

That experience has made me very uncomfortable with multiple check valves--if I could get a 50 inH2O swing in a low-pressure gas line, the pressure waves in a liquid-full line have to be very significant.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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