4 replies to this topic

[processengbd]

Dear respected engoneers I have quite a few question regarding control valve.

 

Usually in datasheet we have to specify different flow rate for specific control valves like at 120% 70% 30% of rated flow. Do we have any guidance of selecting this percentage?

 

We also have to specify the pressure drop and other property data at these different flow rates. Now we usually get the property data from HYSYS but with pressure drop specially I feel uncomfortable in temperature control valve and flow control valve. As for pressure control valve we already know the down stream pressure. Also low pressure drop significantly increase the valve size. So what should be the pressure drop in these valves.

 

We faced some problem in selecting trim type (Equal percentage, Quick opening, Linear). As per GPSA 13th I found that Quick opening is used for on-off operation; Linear for level control & Equal percentage for pressure control. but what trim type can be used for temperature control?

 

Another question is valve mode on instrument failure like (Fail to open or Fail to close) (Fail Safe Position). As far as I know Majority of valve are designed failed to close position except cooling water supply.

 

Another question in valve Cv calculation. I usually do it in HYSYS (Universal Sizing Equation) Then I use GPSA 13th fig no 4-32 to check the Cv to specify the valve in P&ID. Please share your working procedure.

 

 

Now you might be asking usually these things are done by control valve vendor. But before final confirmation from the client about a project we usually do not contact vendor. So the size of the valve is specified by inhouse process engineers.

 

I have attached a hand drawn image for your understating. It will be helpful if you give me suggestion keeping this sketch in mind .

 

This is a 100 MMSCFD Natural gas Teg Dehydratyion plant.

 

I would very much appreciate if you could suggest me a reference or standard regrading these topics.

 

Regards 

processengbd

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Edited by processengbd, 30 March 2014 - 01:36 AM.

[chemsac2]

Sketch seems to have conflicting controls i.e. both flow and pressure controlled. Wellhead is also shown. Never seen a flow control valve on wellhead as choke valve and slug catcher/inlet separator pressure control valve essentially decide flow from wellhead.

 

Hence, would reply in general. Please note that we give flowrates and pressure drops for different scenarios as we want to envelope possible control valve operation. Control valve operation is set by minimum Cv and maximum Cv.

 

Remember general Cv equation i.e. Cv = Q*((rho/DP)^0.5).

 

Q is set by plant operating philosophy i.e. turn-up rate, normal rate and turndown rate. 

 

DP is mostly set by system and not by valve. E.g. in your level control application, pressure drop would be as below:

Pinlet = 1000 pisg - line losses

Poutlet = 90 psig + line losses 

 

Flow affects line losses and hence different pressure drop for different flows. Since absolute pressure values are very high, DP for different flows may not be much different. To explain you that DP is affected by system and not by control valve size, consider no line losses for level control system in your example.

 

For this system, irrespective of your control valve size, pressure drop would always be 1000-90 psig. If you install, say 8" valve, opening may be 50% for given flowrate. If you install, say 6" valve, opening may be 80% or 20% for 10" valve. But pressure drop would always be 910 psig.

 

Hope this clarifies your queries a bit.

 

Sachin

[ankur2061]

processengbd,

 

Your post seems to be written in a hurry. As an engineer it is always a good practice to put things point-wise. Let me try to answer your questions point-wise

 

1. Temperature although a direct measurement from a temperature sensing element indirectly controls the flow of a cooling or heating fluid to control temperature. So in a manner of speaking, a temperature control valve is basically a flow control valve and the characteristics applicable to a flow control vale would be applicable to a temperature control valve.

 

2. Flow control valves characteristics are selected based on the following:

a. Linear characteristics apply when the flow measurement signal being measured by the flow controller is proportional to flow.

b. Equal Percentage characteristics apply when the flow measurement signal being measured by the flow controller is proportional to flow squared.

 

3. Fail position (Fail-Open, Fail-Closed, Fail-Locked) of a control valve is entirely based on the following factors:

a. Direct Hazard assessment either to personnel or equipment if a control valve fails in a particular position. This shall be assessed during a HAZOP review

b.  Secondary hazard assessment due to process upset if a control valve fails in a particular position. This can be either reviewed during P&ID review and / or HAZOP.

c. Process licensor's experience and recommendation based on his supplying many similar processes or technologies to multiple clients. You can argue your understanding with the process licensor, but remember he is the one who developed this process and keeps track of units / plants he has supplied in terms of safety and operability.

 

4. Instrument engineers are the one's who generally get involved in sizing of control valves and not process engineers. A very popular software used for control valve sizing is aptly named 'InstruCalc'. Process engineers provide the process data for the instrument engineer to perform the sizing calculations.

 

Hope this helps.

 

Regards,

Ankur.

[processengbd]

Sketch seems to have conflicting controls i.e. both flow and pressure controlled. Wellhead is also shown. Never seen a flow control valve on wellhead as choke valve and slug catcher/inlet separator pressure control valve essentially decide flow from wellhead.

 

Hence, would reply in general. Please note that we give flowrates and pressure drops for different scenarios as we want to envelope possible control valve operation. Control valve operation is set by minimum Cv and maximum Cv.

 

Remember general Cv equation i.e. Cv = Q*((rho/DP)^0.5).

 

Q is set by plant operating philosophy i.e. turn-up rate, normal rate and turndown rate. 

 

DP is mostly set by system and not by valve. E.g. in your level control application, pressure drop would be as below:

Pinlet = 1000 pisg - line losses

Poutlet = 90 psig + line losses 

 

Flow affects line losses and hence different pressure drop for different flows. Since absolute pressure values are very high, DP for different flows may not be much different. To explain you that DP is affected by system and not by control valve size, consider no line losses for level control system in your example.

 

For this system, irrespective of your control valve size, pressure drop would always be 1000-90 psig. If you install, say 8" valve, opening may be 50% for given flowrate. If you install, say 6" valve, opening may be 80% or 20% for 10" valve. But pressure drop would always be 910 psig.

 

Hope this clarifies your queries a bit.

 

Sachin

chemsac2  Thank you very much for your answer.

 

I am sorry for the confusion regarding the flow control valve at the wellhead. The thing is we have multiple wellhead connected to a header. our client asked us to install a flow control valve in each line.

 

Q is set by plant operating philosophy i.e. turn-up rate, normal rate and turndown rate. 

 

Yes we have used this basis in our projects.

 

DP is mostly set by system and not by valve. E.g. in your level control application, pressure drop would be as below:

Pinlet = 1000 pisg - line losses

Poutlet = 90 psig + line losses 

 

 

Yes we have also specified what you suggested. But my question is will it be possible for a level control valve to simultaneously control the level and pressure. As we dont have any pressure indicator signal input to the controller. Like assume for some time there is increase in supply of liquid in the separator so obviously the level control valve will open more but that is going to increase the down stream pressure (Three phase separator).(though we are using restriction orifice downstream of the valve).

 

Detail of the lines like elbows and fittings will not be available before isometric drawing. So will it be possible to predict line loss at this stage?

 

Regrads

processengbd

Edited by processengbd, 30 March 2014 - 03:31 AM.

[fallah]

 

But my question is will it be possible for a level control valve to simultaneously control the level and pressure. As we dont have any pressure indicator signal input to the controller. Like assume for some time there is increase in supply of liquid in the separator so obviously the level control valve will open more but that is going to increase the down stream pressure (Three phase separator).(though we are using restriction orifice downstream of the valve).

 

Detail of the lines like elbows and fittings will not be available before isometric drawing. So will it be possible to predict line loss at this stage?

 

 

Hi,

 

A LCV cannot control simultaneously the level and the pressure and, as Sachin well explained, the DP and then the downstream pressure is mostly set by the system configuration. Therefore, to specify the exact line loss you have to have the relevant iso drawing, but it can be predicted having the rough distance and elevation diffecrence between the control valve and downstream equipment...