3 replies to this topic

[chemdk]

Hi,

As per API guidelines, Relief valve downstream of a CV is to be sized for the max. Cv (for CV failure case).
However what fluid properties are to be referred to for rated flow calculation i.e. fluid properties at relieving condition or at normal condition.

looking for valuable inputs.

regards

[ankur2061]

chemdk,

The relief valve capacity will be the maximum flow rate through the control valve in the fail open condition. However, the fluid properties will be corresponding to the relieving condition i.e. the relief valve set pressure plus the allowable overpressure (10% over the set pressure for a single relief device and non-fire case as per API STD 520).

Hope this helps.

Regards,
Ankur.

[chemdk]

Ankur,

Thank you for ur reply.

Maximum flow rate thru a CV is when CV is wide open (Max Cv value). However to arrive at max flow rate thru a control valve, shall one use relieving condition properties (density) Or maximum flow is calculated assuming fluid "standard" condition properties.

[ankur2061]

chemdk,

Maximum flow rate occurs due to valve fail open. This could happen even before the line downstream of the control valve gets overpressurized. Overpressure would be a consequence of maximum flow and not vice-versa. Hence maximum flow rate should be calculated at the operating conditions of flow. If the maximum flow rate due to valve fail open is defined as mass flow and you need to report the flow as volume flow then consider the lowest density of the fluid at the operating conditions (temperature / pressure) to arrive at volume flow @operating conditions.

Regards,
Ankur.