Pressure drop - low quality steam at low pressures

[Drexl]

Hi,
I was wondering if anyone know a proper pressure drop correlation for
1. Low quality steam x=0.1-0.3
2. Low pressure 0.2-1.5 bar(a)

I have a flash tank connected to the condenser by a short pipe. At a disturbance case I expect extreme flow to the tank and would like to calculate what the pressure in the tank needs to be to push out the two-phase steam.

Drex

 

[Drexl]

No process engineers out there?

 

[Latexman]

This may help you:

http://www.google.com/url?sa=t&rct=...uoTwAw&usg=AFQjCNG--G1f9h420-zUKIOPupdxgMSMgg

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[katmar]

This is one of those problems for which there is no single "right" answer. A good start is to plot your range of possible conditions on a flow regime map. Then apply all the correlations you can find that apply to the flow regimes covered. To do this you may also have to define whether the flow is vertical or horizontal. The range of answers you get will give you an indication of the reliability/accuracy of the correlations in that range.

In situations like this it is good to remember than an engineering design is a decision and not a calculation. You do all the calculations that you think will give you useful information, and then you apply your engineering experience and judgement to decide on the solution. In this case the decision may even be that there is no reliable way to predict the pressure drop and that it is necessary to go back a step and fix the upstream problem that is giving rise to the 2-phase flow.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[BigInch]

Unwin Equation
Fritzche Equation
Spitzglass Equation (saturated steam < 200 psig)

Can all be used for typical steam flow.

Now, given that allowing slugs of liquid to fly about a piping system can actually be dangerous, would it not be better to ensure that the condensate be drained from the condenser by other means... a drain valve for example.

Contrary to popular belief, everything in this world does not require a process engineer. Thermodynamics is usually an ME topic. At least I learned it in the ME school of things.

 

[Drexl]

Latexman, thanks. I will review this. There is quite a lot of literature similar to this that investigate mass flows from pipe breaks connected to the pressure vessel of nuclear reactors. The problem is that they have high pressure and usually saturated liquid at inlet. The translation to my case is not so easy.

katmar, since I have very high velocities and high void it will be mist flow or in some cases perhaps annular mist. Likely some critical two phase model would be most useful. Since this is a exceptional disturbance case and existing installation I don't want to redesign unless absolutely necessary. A proper separation of steam and condensate for this case is not reasonable I think as flow is so much higher than at normal operation.

BigInch, the listed correlations are to my knowledge used for single phase pressure drop. With steam quality this low the homogeneous density will be 3-10 times that of the pure steam and the viscosity for calculating Reynold number needs some adapted equations. If you have used 1-phase pressure drop correlations for 2-phase with only compensation for density and viscosity I would be very interested. Do you have any reference for this. The installation investigated has a few drainage valves from preheaters via a flash tank to the condenser. The condensate will always flash heavily in the control valve so the flow to the flash tank will always be low quality steam. The flash tank has a steam pipe and a condensate pipe connected to the condenser. At this disturbance case the mass flow will be so high that proper separation is unlikely and the condensate U-tube will likely blow out and the 2-ph. steam will blow out both through steam and condensate pipe.

 

[BigInch]

Exactly my point. Steam piping designers spend a lot of effort on keeping their steam dry.
Shouldn't you be preventing two phase flow and condensate blowout.
If so, why do you think not?

http://www.kirsner.org/pages/condInduceWatHamText.html

 

[Latexman]

In the relief device field, HEM is used extensively because it is said to be always conservative. My company calls it "Recognized and Generally Accepted Good Engineering Practice (RAGAGEP)." Why not use HEM?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[stgrme]

Perhaps, the solution to this problem is to use a more typical arrangement for the connection of a flash tank to a condenser. This typical arrangement is to have two connections from the flash tank to the condenser. A connection from the top of the flash tank will carry saturated steam to a point on the condenser above the tube bundle. A second connection from the bottom of the flash tank will carry saturated liquid to a point on the condenser below the tube bundle, but above the liquid level. With this arrangement, each connection handles only a single-phase flow.

Best of luck!

 

[BigInch]

As it should be.

 

[Drexl]

I see I was a bit unclear in my post. There is of course steam exhaust pipe and a liquid drain pipe. During normally occurring operating cases sufficient separation will take place. It is however possible to have all valves draining to the tank simultaneously giving extreme flow to the tank. The probability of this event is very low.
HEM looks interesting and I would be interested in a textbook describing it in detail (google doesn't give anything of substance).

 

[Latexman]

Did you look at this reference?

Select the Best Model for Two-Phase Relief Sizing

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.