8 replies to this topic

[ayan_dg]

The sitution is

There is a let down station which reduces the pressure of HP steam (45.9 kg/cm2 g , 385 C ) to LP steam (4.1 kg/cm2 g , 351 C ) by means of Pressure Control Valve. the pressure control valve takes feedback signal from downstream of the let down station. To protect the LP lines after the Pressure control (reducing) valves PSV is to be installed set at 6.1 kg/cm2 g.

I have to calculate thre relief load for various possibilities.

The PCV/ Pressure reducing valve is fail close. (It closes on instrument air failure, electric supply failure) then my question is

1) Should I consider inadvertant opening of the valve to calculate relief load ?
2) should I consider the valve stuck open case ?

If yes , then how to calculate the relief load. I am sizing the valve in such a way that at maximum demand of steam it is 90% open. As I do not have the CV characteristic curve at this point of time should I linearly pro-rate it to 100% open condition to get the relief load

[benoyjohn]

Dear,

You will have to consider both the inadvertant opening of the valve and the valve stuck open case.

1. You will have to first estimate the full open CV of the valve you are going to provide. Linear pro-rating should work in this case, once you calculate the maximum required CV.

2. The worst case flow through the valve then need to be calculated. While doing this, consider the upstream pressure of the valve as the maximum operating pressure possible upstream of the valve and the downstream pressure as the relieving pressure of the CV downstream PSV. This will give you the required relief load.

Regards
Benoy

[fallah]

Dear,

You will have to consider both the inadvertant opening of the valve and the valve stuck open case.

1. You will have to first estimate the full open CV of the valve you are going to provide. Linear pro-rating should work in this case, once you calculate the maximum required CV.

2. The worst case flow through the valve then need to be calculated. While doing this, consider the upstream pressure of the valve as the maximum operating pressure possible upstream of the valve and the downstream pressure as the relieving pressure of the CV downstream PSV. This will give you the required relief load.

Regards
Benoy

Could you please clarify your mean about "downstream pressure of CV" in worst case for calculating relief load?
Regards

[benoyjohn]

This is the pressure expected downstream of the Control valve during the relieving scenario.

In this particular case it works out to be 6.1 x 1.1 = 6.71 kg/cm2 g.

Regards
Benoy

[JoeWong]

Benoy,
Good response. The concept and methodology are correct.

Just wanna to add my opinion...
Control valve can be quick opening, linear, equal percentage or other. Assuming you assigned the control valve to operate at 75% opening at normal design flow and design pressure drop, the maximum CV for selected control valve can varies significant to the valve characteristic. E.g. a equal percentage control valve, the max CV can be 300% of the normal CV.

Thus, you may have to decide what type of trim you will use, the operating opening that you would like to operate the control valve (this will define the max CV) and last select a good control valve for the maximum CV. Calculation prior to this...the results could vary very much...

[benoyjohn]

JoeWong,

Thanks. I agree with you.

Regards
Benoy

[djack77494]

I think this was implied by JoeWong, but allow me to stress this point. During the design of a project, the process engineer will need to complete a datasheet for any new or modified control valves. Later he may have to size relief valves, and the sizing of some fo the relief valves may be dependent on the flowrate through some of the control valves.

Know/be aware/use caution: The final, selected control valve may well be larger (i.e. have a larger Cv) than the control valve anticipated by the process engineer. Do not assume that the instrument engineer will buy a control valve with the Cv that you need. He/she applies their own overdesign factors to what you think you need. Then they look for a valve to do the job. Naturally, they will not find an exact match, so they pick the next larger valve. What is finally installed may be capable of passing much more flow than you anticipate. Lesson: Communicate with your instrument engineer.

[JoeWong]

Doug:
You probably have stressed the right thing i have mentioned earlier.

ayan_dg:
Off the topic, is the study a school assignment - or a real world application ?

Further extention on Doug point...
In many events, we can not afford to wait till Instrument engineer to select and specify and the project / client to decide a particular model; two approach may be taken :

i) take it as case study and work together with instrument engineer and Control valve vendor to define & freeze the model up-front

ii) make an effort to pre-define the maximum CV, spell out the allowable maximum CV in datasheet and let the instrument engineer and vendor find a "good" Control valve.

[ayan_dg]

thanks for all the good suggestions.

It is actually an assignment given to me & not a real world appliation