ARVs and Check Valves - Actuation

[jdogg05]

I was reading on how a ARV (automatic recirculation valve) works and came across the following web page:

http://www.wermac.org/valves/valves_arv.html

It shows a cutaway of a valve that is similar to the one installed on the discharge of the LP BFW pumps in our plant.

What I am having trouble understanding is: how is a check valve actuated? I understood it to be actuated by flow (because there is very little pressure drop across a properly spec'd check valve; therefore, differential pressure can't be doing it...). With the ARV, if the check valve is flow actuated, how is the passive control accomplished? i.e., as I understand it, when the pump is running the flow will always keep the ARV open to the process flow piping (not the recirc. line). The only time the flow would be directed to the recirculation line is when the pump is shut off... Why does it need to be recirculated then? Is it maybe that the ARV under full pump operation is actually open to the recirc. line a bit? Isn't that inefficient?

 

[pennpiper]

No wonder you are confused. I was too. There are a lot of complex words and terms in the link showing the cut-away.
I will try to simplify it.
In the cut-away:
A- The inlet from the pump is in the bottom nozzle
B- The outlet of the check valve (ARV) is out the top nozzle. This is the normal flow.
C- The Recirculation line (Also sometimes called a Minimum Flow Bypass) is the smaller connection out the nozzle on the right side of the ARV.

Now, if for some reason the "Normal" flow out the top nozzle (B) is blocked downstream somewhere then the recirculation nozzle (C) is opened and the (now) excess flow is routed back to the pump suction tank or vessel.

It is all automatic.

prognosis: Lead or Lag

 

[BigInch]

an ARV acts simply as a relief valve going to suction.

A check valve is activated by "no flow", which would no longer supply sufficient force from fluid impact to hold the clapper, or ball, in the open position.

Independent events are seldomly independent.

 

[jdogg05]

I also noticed after looking at the piping that there is a check valve after the ARV on the recirc. line (also a isolating gate valve). This makes sense to me. This way you can't have back flow from the recirc. line into the pump discharge. There is also a gate valve on the main discharge line; however, no check valve. BUT, the ARV actually serves two purposes: automatic recirculation, and it acts as a check valve to prevent process backflow into the discharge.

 

[LittleInch]

Try this website and scroll down a bit - makes it a bit clearer.

http://www.mento.no/articles.php?sub=242

As the main disc and inner shaft rises up, the orifice / opening to the re-circ line is gradually reduced unitl it is fully closed off.

For these valves the check valve is actuated by the spring force which closes it and then is sealed close by the pressure difference accross it.

Why does it need re-circ? Centrifugal pumps don't really like operating below a certain flow and some types, especially axial pumps can sifnificnatly increase pressure and power at low / no flow, so some device is required to maintain flow at some flow (10-20% of max flow is quite common) if the outlet is blocked or outlet pressure rises for some reason.

ARVS are design individually so that once flow through the main line exceeds the minimum re-circ amount, the re-circ line should be auto isolated otherwise yes, it would be inefficient. Some older systems did exactly that, i.e. they let a fixed amount recircualte balck regardelss of the flow throught the pump.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way