Decreasing flow at higher opening of valve

[virk]

A customer of us describes the following problem:

In an ammonia (absorption) refrigeration installation one expansion valve group consists of shut-off valve DN25, control valve DN25, shut-off valve DN150 (expansion has taken place).
Between first shut-off valve and control valve a pressure gauge (manometer) is installed.
Normal "high" pressure of system is 12 bara. If this control valve is in normal working position the installed manometer shows approx. 10 bara. So we already have a pressure loss of 2 bars due to thin lines and long length.
Customer now says, if I completely open my control valve, net flow into the separator decreases, so I limited the valve to 80% opening.

At this moment after I thought about this problem some time, I am unsure whether customer perhaps misinterpreted something.

What do you think? Under which conditions this phenomenon can occurr, decreasing flow at higher valve opening?

Kind regards

 

[RubenJ]

This is called choked flow, it happens when the pressure at the valve vena contracta(the smallest actual flow area) drops belob vapor pressure, this causes formation of bubbles which restrict liquid flow; in the case of gases, it happens when gas flow reaches sonic flow velocity.

 

[scalleke]

Ruben,

Does this mean it could be solved by partially closing the DN 150 downstream block valve?

i.e. opening the control valve to 100% and partially closing the DN 150 valve?

Thanks,

Scalleke

 

[hydrae]

virk
How is the flow and pressure measured, can the position of the valve affect the measuring devices?
or
is heat added in the long thin lines such that the liquid turns into vapor in transit with a lower backpressure?
Hydrae

 

[virk]

hy drae

Flow is "measured" by watching the level of the large suction vessel after this expansion line. While external conditions remain the same level is decreasing in this particular case (I myself did not check this behaviour, it is a description of a problem of one of our customers).

Nearly no heat is added in the long thin lines, but we know that this line is causing a pressure drop which is not negligible.

Ruben, scalleke

we also thought about pre-flashing of the ammonia before control valve. But in this case I also think that scallekes consideration of closing one valve to increase flow again should be examined.

I will search for "choked flow", because I would like to understand this phenomenon completely before I tell anything to the customer.