3 replies to this topic

[dhns]

Dear sir,

To control a flow in a pipeline. whether i put a flow sensing element(ie. FT along FIC) before the control valve or after the control valve ?

is the control valve operated by two signal (flow value and liquid level of the tank)?



Thanks in advance.

dhnsekaran

[katmar]

I don't believe there is a fixed rule about where the sensing element should go relative to the control valve. It is probably more usual for the element to be upstream of the valve, but I am currently looking at a vapor flow control installation where I intend putting the element downstream because the pressure is more constant there.

If the tank level is important then you can control it and let the flow find its own value. It is possible to use a cascade control system where the tank level sets the setpoint for the flow controller, and the valve then controls to this flow value, but that would be unusual (in my experience).

[djack77494]

While there is no "right" answer as to where to locate the sensing element relative to the control valve, there ARE more common and less common solutions. The most common configuration is for the flow measuring orifice to be located upstream of the flow control valve (or level/temperature/pressure control valve). In my opinion, the main reason for this is that it is far easier to get a good flow measurement upstream of the control valve. Here the straight length of upstream piping is usually easier to design. Downstream of a partially open control valve you have about as bad a flow pattern as could possibly exist, and the maximum amount of straight pipe is needed to regenerate fully developed flow. Also, generally conditions upstream of the control valve are more constant than they are downstream (although not always, as Harvey has demonstrated). Especially for vapor flows, this is an important consideration. As a final consideration, there are situations where a second phase may develop inside or just downstream of a control valve. This could be due to flashing of the liquid, or just entrained gases coming out of solution. A large pressure drop as might happen at a control valve is just the spot for this to occur. We cannot accurately measure the resulting two phase mixture using the standard orifice meter, but we could if we had positioned it upstream of the control valve before the vapor phase developed.

Doug

[JoeWong]

Katmar / Doug,
Good advices...

Apart from :
i) Constant operating parameters
ii) steady fluid properties i.e. single phase

i would suggest :

iii) Pressure Rating
High pressure rating of flow element will be costly. Thus, it is always prefer to locate the flow element at the system with lower pressure rating, downstream of control valve.

iv) Vibration
Pressure drop across control valve would partially convert its energy to noise and vibration. The vibration (higher intensity) would travel along fluid flow direction. Some flow element (i.e. Coriolis,Vortex) is sensitive to vibration. Vibration sensitive flow element is always advisable to locate upstream of control valve.


dhns,
A little more discussion in "Flow Element (FE) Upstream or Downstream of Control Valve (CV) ?"

Good luck.