TRV arrangements around meters, control valves and loading arms

[Michael_C]

I'm considering a bulk petrochemical storage facility with road tanker loading facilities. For TRV arrangements, we direct product back through thermal relief bypasses across block valves, with product being directed back to tank. There is a scenario where the product being loaded into road tankers has passed through a meter, a control valve (controlled by the batch controller) and then through the loading arm into road tanker. The meter and control valve are fitted flange-to-flange, and then into the loading arm. My questions:
(1) Is thermal relief required across the control valve and meter arrangement to prevent thermal pressure buildup in the loading arm?
(2) Is it permissible to relieve from downstream to upstream of such a meter - are the legal / compliance restrictions on this?

Your views would be appreciated (please reference codes and standards as far as possible). Thanks all.

 

[LittleInch]

Thermal relief is a relatively small amount but you need to be able to quantify the amount, if any, which returns tot he tank after it has been metered.

An unscrupulous operator could divert lots of product back to the tank. So you might get pulled up if the system is audited by a customer or some third party organisation which "certifies" your system as being secure in terms of metering if you can't show a means of quantifying the thermal release or to notice if e.g. the relief valve starts passing.

Depends on the volume and start / finish temperatures are for the locked in section, but some liquid fuels do expand quite a bit and the ROT is about 3 bar per degree C rise.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RVAmeche]

Hard to say without a sketch, but typically control valves control flow to users/storage. So generally the upstream piping TRV cascades backwards into the supply tank and piping downstream the control valve is "open" to the end user/storage.

I wouldn't route TRV stuff around control valves or locations with big pressure differentials, it's probably asking for trouble or components to get mixed up.

 

[georgeverghese]

A TRV would be required here if there is a block valve downstream of the control valve.
Thermal TRVs' is such applications are arranged in series to cascade back in to the source tanks, where the setting of each TRV is progressively lower as you proceed upstream toward the source tank. So the TRV furthest downstream has the highest setpoint. Use balanced bellows thermal TRV in such cases - these can tolerate up to 80% backpressure without affecting the TRV setpoint.

 

[RVAmeche]

Also note that if this is ASME piping all TRVs need to be ASME relief valves. The military installations I dealt with were very against this and only the device into the storage tank was a real PSV. Everything else were regulators that looked like a PSV and was field adjustable.