13 replies to this topic

[prajot]

Hi,
Can we operate two control valve in series?
In case of refinery we are normally having pump discharge control valve in the source unit and in addition to it, we are having a control valve in the receiving unit.
In such configuration I think one valve is redundant.

[pleckner]

We need more details.

What are these two control valves controlling? Perhaps they are redundant, perhaps not. They may be fighting each other.

Again, we need more details.

[wnovaes]

Prajot,

This kind of configuration with two controls valves in series is not frequency used. However, we can find some particular applications for it. A case is the use of a pilot-operated control valves in a critical application. The performance of a pilot-operated control valve is not easy assessed and its unexpected fail might result in troubles. For example, in a boiler natural gas feed and with a pilot-operated control valve pressure controlled is recommended the use two pilot-operated control valves in series. The first valve is set at normal operation pressure and second one set slightly higher than normal operation pressure. In this arrangement, the second valve works monitoring the performance of the first one.
In another hand, for air-operated control valves I don’t know any application. Please, I am not saying it there isn’t. However, I can’t understand the needed of two air-operated control valves in series for any application.

Best Regards,
WSN.

[fallah]

What i know about this subject is: when we need to control the pressure and flow in a line,simultaneously,we shall use two control valves in series, because it is not possible with one valve.
regards

[prajot]

thanks for response
Actually this configuration is at more than one place in this project.
In one case as fallah mentioned we have flow and pressure control valve in series and in other case we have two flow control valve in series.
I am happy with first configuration as there is no other better way to operate the system than to provide two CV in series. In other configuration we have Atmospheric gas oil coming from crude unit which is feeding to DHDS unit, this line has one flow control valve in crude unit and other flow control valve in DHDS unit. I feel in such case one valve will have to be fully open in order to control the system.
In another situation we have rundown diesel product line which runs to blending header. This line has Flow control valve cascaded with column level control, plus one another FV in blending header.
Give some thoughts.


Regards

Prajot

[JoeWong]

We need more details.

What are these two control valves controlling? Perhaps they are redundant, perhaps not. They may be fighting each other.

Prajot,
There will be many combination of control loop with double control valves...Please provide more details e.g. P&ID or sketch, control loop, etc...

Without these, no way people can comment nor advise.

JoeWong

[Milutin]

 ADU__HDS.jpg   34.3KB   113 downloads

Hi all,

Connecting diesel flow from ADU to HDS can be designed as on attached picture. In this way ADU operator is allowed independently to adjust diesel cut and HDS operator can adjust unit throughput independently to ADU diesel flow (up to limit when take all diesel stream).
In this configuration pressure controller redirect extra diesel to tank.

In configuration whidout pressure controler LC on HDS drum is on manuel and opened (out of operation), if it is on auto it will change diesel cut point.

Regards,

Milutin

[djack77494]

The simple reply to your initial inquiry is that you can have only as many control valves as you have independent variables (or degrees of freedom). You cannot effectively control the flowrate of a liquid out of a vessel, for example, using one outlet line with two control valves. They will "fight each other" since they are both attempting to manipulate a single independent variable. Change the fluid to a gas, and you can do it because you have introduced another independent variable, which is the pressure of the gas in the reservoir between the two control valves in series. Likewise if you put a partially full tank between two valves in a liquid system. It's all about degrees of freedom and avoiding overspecifying your system.
Doug

[JoeWong]

The simple reply to your initial inquiry is that you can have only as many control valves as you have independent variables (or degrees of freedom). You cannot effectively control the flowrate of a liquid out of a vessel, for example, using one outlet line with two control valves. They will "fight each other" since they are both attempting to manipulate a single independent variable. Change the fluid to a gas, and you can do it because you have introduced another independent variable, which is the pressure of the gas in the reservoir between the two control valves in series. Likewise if you put a partially full tank between two valves in a liquid system. It's all about degrees of freedom and avoiding overspecifying your system.
Doug

Doug,
You are right...your response triggered me a term "Degree of Freedom analysis". This probably helps to identify redundancy / catching / fighting of multiple control loop and final control element for a complicated control system.

JoeWong

[prajot]

From above replies I can summarize my understanding as "Two control valves can operate in series, provided they are controlling two independent variables e.g. pressure and flow." "Control valves in series controlling the same variable makes no sense."
Thanks for replies

[Steve McGahey]

From above replies I can summarize my understanding as "Two control valves can operate in series, provided they are controlling two independent variables e.g. pressure and flow." "Control valves in series controlling the same variable makes no sense."
Thanks for replies

I don't think you can ever justify having two control valves in series.

My reasoning?

A valve alters the pressure drop in a bit of line, to change the flowrate.

Say what you want about what it is changing, and what it is controlling, but this is the ultimate physical reality.

So, having two valves which are being actuated, in series, is redundant.

Perhaps you really want a cascaded control system - a pressure controller which manipulates the setpoint of a flow controller... ?

[djack77494]

Steve,
I have used two (pressure) control valves in series for a case where we had a need for a very low pressure blanket gas being fed from a very high pressure process gas system. The feeling was that a single stage pressure reduction would not suffice. I would be interested in hearing comments back on just what sort of range might reasonably be expecteed from a single control valve (and hence, when a second valve might be required).
Thanks,
Doug

[Genii]

I don't think you can ever justify having two control valves in series.
.
.
.
Perhaps you really want a cascaded control system - a pressure controller which manipulates the setpoint of a flow controller... ?

I have seen two control valves in series one is controlling level of buffer vessel located at inlet line and the other is located at the outlet to control the flow of Ammonia to urea reactor.

Similarly have also seen cascaded system, level controller changing the set point of flow controller. Thats depend upon the scenerio.

Also I have also heard that if u r going to drop very high pressure to very low pressure then u may have to use two control valves in series, any idea?

Regards.

[JoeWong]

I don't think you can ever justify having two control valves in series.
.
.
.
Perhaps you really want a cascaded control system - a pressure controller which manipulates the setpoint of a flow controller... ?

I have seen two control valves in series one is controlling level of buffer vessel located at inlet line and the other is located at the outlet to control the flow of Ammonia to urea reactor.

Similarly have also seen cascaded system, level controller changing the set point of flow controller. Thats depend upon the scenerio.

Genii
I believe the original post is two control valves in series in a line

Also I have also heard that if u r going to drop very high pressure to very low pressure then u may have to use two control valves in series, any idea?

djack77494 post (one post earlier than yours) is exactly mentioned this...

JoeWong