steam pressure drop through a control valve (evaporator case)

[Kdolph]

Hi Everyone,

This is a situation about the steam pressure drop at the steam feeding of a 3 effect evaporator.

The steam for various process of the plant is produced on another 1 effect evaporator, and the pressure of this steam is 2.6 bara (measured). Part of this steam is used to evaporate juice in a 3-effect evaporator.

The steam is supplied by a 6” pipe, like is shown in the next picture:

http://files.engineering.com/getfil...14e-b9ccf3e2148c&file=steam_to_evaporator.png

The control valve datasheet is shown in the next link:

http://files.engineering.com/getfil...4acd-86e9-f566c77e2fc2&file=control_valve.png

I want to estimate the heating supplied to the 1st effect of the evaporator. When the control valve is at its 100% of lift the steam pressure upstream of this valve is 0,8 bara. So the question is. Why this pressure is less than the 2.1 bara (showed at maximum flowrate of the control valve; 98,7 lift). What am I missing?.


Thank you for you time.

 

[ione]

What's the length of the 6" pipe from the steam source at 2.6 bar(a) and the inlet of the control valve?

 

[Kdolph]

the length of the 6" pipe is around 25 m, and the inlet of the control valve is 4"

I have done the pressure loss calculation and is negligible at 4000 kg/h of steam

http://www.tlv.com/global/TI/calculator/steam-pressure-loss-through-piping.html

 

[ione]

Just to be sure: is the pressure upstream the control valve really 0.8 bar(a)? Have you measured it with a vacuum gage?

 

[Kdolph]

it's absolute pressure. I'm pretty sure of that. For example when the plant is stopped the measurement is around 1000 mbar (seen at the control room)

 

[hacksaw]

well you have basically answered you own questions: the valve is wide open and the inlet pressure has dropped below atmospheric...you need to look at what is limiting the flow upstream

 

[Kdolph]

yet, it doesn't make sense to me.
-The pressure at the valve inlet should not be a bit less than 2.6 bara, considering the friction losses are negligible.
-Based on the valve Data Sheet the maximum pressure drop across the valve is 0.5 bar. So the minimum pressure at the valve outlet should be 2.6 bara - 0.5 bar = 2.1 bar?
-Why the reading is 0.8 bara?

 

[ione]

Can you read on a gage the value of the pressure at the valve outlet, or is it just a value speculated on the base of the valve's specs? have you the possibility of reading the steam temperature at the valve inlet?

 

[Kdolph]

Sorry I made a mistake in the first post
"When the control valve is at its 100% of lift the steam pressure upstream of this valve is 0,8 bara.". It's really downstream the control valve where the pressure is measured.
a draft of a P&ID of the steam line is in the next link:

http://files.engineering.com/getfile.aspx?folder=265b240f-037a-40a1-a608-b4bbcfd9ace0&file=PID.png

So, the only measurements are the inlet valve pressure (pressure indicator) and the valve outlet pressure (pressure transmitter). There's no temperature measurement.

 

[ione]

So resuming you do have steam at 2.6 bar(a) pressure at the control valve inlet and at 0.8 bar(a) pressure at the control valve outlet….. Uhm, I’m still thinking you’re taking relative pressure for absolute pressure. If you had the possibility of sticking a thermometer on the pipe and then measuring the steam temperature this could dispel some doubts.

 

[racookpe1978]

Could he be getting "superheated" steam right after the valve if steam temperature does not drop across the control valve, but steam pressure goes down substantially through the valve?

 

[MortenA]

IMO you have either a valve or a PT that is not in working order - or a much higher flow than you expect? Do you have any way of measuring the temperature downstream the CV (of the steam). check with an isenthalpic steam chart (Mollier diagram) - the P/T should match uo or there is something wrong with the measurements.

 

[ione]

racookpe,

From a theoretical point of view it could be possible to have what you've described, but in real world applications, where steam is never ideally saturated (its quality is below 100%) the result of the steam passage through the control valve is very hardly that of having a superheated steam downstream the valve, but rather it’s that of having an increase of steam quality (indeed very small in this specific case, taking into account the quite low pressure reduction).