3 replies to this topic

[bhaskaran]

In sizing of pressure safety valve ulimately we are finding the orifice area .Orifice area was chosen as per the ASME section VIII guidance( D,E,L,M)

The orifice area for L = 2.853 SQ.Inches & size (in x out) = 3 x 4

dia of orifice from the above orifice area comes to 1.9 inches only (arriving dia from Pi/4*d2 = 2.853 in2)

i have following clarification
i)In safety valve orifice area represents what?
ii)Orifice dia comes only 1.9" whereas inlet safety valve flange size was 3"?

Kindly clarify

Regards,
Bhaskaran.r

[ashetty]

Hello Bhaskaran,

Please see figure 2 or 3, on pg 6 & 7 of API RP 520 or any good sketch of PSV internal. With reference to this,When the operating pressure reaches the set pressure, the PSV disc will lift and relieve through the outlet.

We need a certain area of the nozzle to achieve the required relieving load. Actually it is the area of the nozzle which is called the Orifice area. Based on this orifice area the appropriate valve should be selected from the latest Crosby or Farris (or other vendor) PR valve catalog or API RP-526, designated in alphabetical order as mentioned by you.

Regards,

Shetty

[fallah]

In sizing of pressure safety valve ulimately we are finding the orifice area .Orifice area was chosen as per the ASME section VIII guidance( D,E,L,M)

The orifice area for L = 2.853 SQ.Inches & size (in x out) = 3 x 4

dia of orifice from the above orifice area comes to 1.9 inches only (arriving dia from Pi/4*d2 = 2.853 in2)

i have following clarification
i)In safety valve orifice area represents what?
ii)Orifice dia comes only 1.9" whereas inlet safety valve flange size was 3"?

Kindly clarify

Regards,
Bhaskaran.r

Do you expect the orifice area diameter would be the same as that for valve flange?If so,you can not title it as orifice!
On the other hand,taking the same size as valve flange may lead to have huge valve body increasing the cost,reaction force,...!

[JoeWong]

i have following clarification
i)In safety valve orifice area represents what?

Whenever engineer calculate the "area" for PSV, the calculated area is REQUIRED EFFECTIVE DISCHARGE AREA (A_E,R). Then PSV with STANDARD EFFECTIVE DISCHARGE AREA (A_E,Std) will be selected such that

A_E,R < A_E,Std

*based on same Kd factor. If diff. Kd factor use, it shall be readjusted...

The STANDARD PSV is designated as D,E,F... with relevant STANDARD EFFECTIVE DISCHARGE AREA (A_E,Std). One shall remember the standard could be ASME, API, GB, or other standard which may have different STANDARD EFFECTIVE DISCHARGE AREA (A_E,Std).

Now... let focus on one standard (say ASME). All PSV manufacturer will design their PSV to meet the STANDARD EFFECTIVE DISCHARGE AREA (A_E,Std) (by ASME) so that they can be endorsed as ASME compliance PSV (by ASME). As each PSV manufacturers will have different level of technology, model, material, etc, there will be varieties of PSV design.

ii)Orifice dia comes only 1.9" whereas inlet safety valve flange size was 3"?

If you refer to any PSV, you will see that fluid is passing the flange follow by a nozzle, bounce back from the disc, before it get into the discharge chamber.

Flange may have diameter of A
Nozzle may have diameter of B
"Effective diameter" back calculated from STANDARD EFFECTIVE DISCHARGE AREA (A_E,Std) (subject to standard) may have diameter of C

Hope this help.