sensor and cv

E

Thread Starter

engineer

hi all,

when to place cv downstream to sensor and when upstream to it. i mean for all the 4 variables (flow, temp, pressure, level). a reference to a easily available literature would do. a rule of thumb would do more.
 
B

bob peterson

By CV do you mean "control valve" as in automatic modulating valve? Or an automatic on/off valve? Or an isolation valve?

Generally, one puts the sensor where one wants to make a measurement.

One might be inclined to install isolation and bypass valves on an instrument to allow for easier maintenance of the instrument.

--
Bob
 
control valve

lets put it this way, say we are doing simple level control, then the level transmitter measures the level of the particular tank. now the control valve can be installed both ways, upstream and downstream to the tank. in the upstream case it will allow the liquid to come and in the downstream case it will allow the liquid to go.

how to decide which one is better for a particular case? or have i missed something completely?
 
B

Bruce Durdle

If you are using level control, you will have a variable uncontrolled flow that is a load on the system. If it's a header tank, the load will be the draw-off from the tank to the end-users. If it's a waste sump, the "load" will be the run-off from whatever sources are feeding it. You do not have any control over this flow and in fact it can be considered the "problem" your control system is intended to solve. In the literature it is often referred to as the "wild" variable.

For level control, you have to adjust the value of the manipulated flow stream so that the average value of the total of inflows + outflows is maintained at 0 over some suitable time period.

If the wild flow is into the tank, you must control the output (unless you have a really weird setup where you have a controllable inflow that you can use to make up the total inflow to match a high level of load). Similarly, if the wild flow is the tank draw-off, you must manipulate an inflow. You don't usually have the option of picking one or the other - it's set by the piping arrangement of the vessel.

Bruce.
 
Hello

the place of control valve depend of your installation: how the liquid of your tank is managed at the outlet (by gravity & pressure or with a pump)?

You can't have a control valve connected to outlet of the tank if you have a pump downstream because may be you will have a problem of miniflow. In this case, you can see some installations with a control valve installed from inlet of the tank. Now with gravity and/or with pressure, you can have control valve at the outlet of the tank etc.

This is a process topic and the choice of upstream or downstream is done according process approach.
 
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