×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

(OP)
Hello. I am working in a Project in which we have several safety valves. As it is well known, safety valves discharge creates transient forces on each change of direction due to the pressure wave generated when opening. Once the discharge is stabilized, there is also a stationary load at the point of discharge due to the "jet" effect.
When the valve discharges saturated or superheated steam the calculation of the stationary load can be found for example in Appendix II of ASME B31.1 and the transient loads can be calculated using papers like G.S.Liao's one "Analysis of Power Plant Safety and Relief Vent Stacks".
However, in the case we have biphasic fluid, these methods do not seem appropriate. I would greatly appreciate if somebody can give me a hand of how to calculate these forces (transient and stationary) in this case. Also, if anybody know of a software which calculate this, similar for example the way AFT Impulse does for water. Thanks in advance.

RE: Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

The decompression effect and accompanying expansion of gas slugs can accelerate liquid slugs, however the maximum force available to do so is the maximum pressure in the pipe. As the gas expands, pressure is reduced to whatever the exit pressure is, so the maximum force available to cause any such acceleration is the maximum Pressure x Area of the pipe.

Independent events are seldomly independent.

RE: Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

Kind of depends what your two phase fluid is. If the initial liquid part can vapourise to some extent as the pressure rapidly decreases coming out of the valve, this adds vapour to the initial vapour mass. You might find that the velocity and hence force from the liquid elements becomes very large, which is why liquid knock out pots are common in such locations. Avoiding two phase relief is much better than trying to deal with it afterwards.

My motto: Learn something new every day

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

RE: Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

(OP)
LittleInch and BigInch, thank you for your interest. I agree that the best is to avoid dealing with two phase flows, but sometimes is not possible. We have safety valves protecting high pressure heat exchangers in case of tubes failure. In this case, a big flow of hot water would be released through the safety valve and it will flash. Looking at the literature I have found, the result is that there is not a clear understanding of the forces created by a two phase fluid (neither transient nor stationary ones). It seems not easy to know if the fluid can be assumed as bubbly, slug, annular,... and there is not formulation. I have seen two softwares which seem interesting and well known (Pipenet and Bosfluid/Dinaflow) but none of them work with 2 phase fluids. Logic says that there will be forces created during the transient and stationary phase which could do that the piping supports in the safety valve discharge pipe be damaged (or broken) and even this should be taken into consideration when determining the pipe design pressure and pipe thickness. Any help?

RE: Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

Even though the initial force is P x A, the force that can be exerted by a moving slug when stopped is dependent on how long that P x A has been acting and accelerating the slug and what velocity it has when it is brought to a stop. Chances are that as the distance over which the slug travels increases, the pressure decreases, but the velocity of the slug increases, due to the acceleration forces acting for a longer time. It is also true that the pressure may not decrease, if fluid continues to be feed at a rate that can keep up with the expansion of the gas. That's why I said the maximum case is with full line pressure accelerating the slug. You could look at possible resistance to that as the backpressure x Area as applied at the end of the line, but if it is to a vent, that would normally be almost zero.

Normal practice is to knock out and hold the liquids in a flare scrubbing pot before sending the gas on to the vent. In fact great effort is made to keep liquids out of a vent or flare line. I would suggest that you do it that way. If you can't, make the pipe strong enough so that you don't have to vent to keep the pressure lower than its design limit.
http://www.exterran.com/Products/production-equipm...
Trying to handle liquids and gases in a highly dynamic venting situation can get dangerous very quickly. Sooner or later the pipe will most likely move a lot.

Independent events are seldomly independent.

RE: Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

(OP)
BigInch, as you know there are situations where you cannot avoid having two phases. For example, as I said previously, relief of high pressure hot water. Water will flash. And as you say, I am worried of pipe supports, to avoid pipe movements due to transients. There are softwares which deal with 1 phase fluids, but I do not know of any dealing with two phase fluids to determine the forces in these transients. Do you know of any?

RE: Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid



Fine, but why can't you vent it after it's flashed. Must it flash inside the vent pipe?

No. I probably wouldn't believe them, if there were any.

Independent events are seldomly independent.

RE: Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

elarriba,

First,make sure you actually have two phase flow. If your shell is at high temp and under pressure it may well just evaportate and you end up with "heavy" gas with only very fine particles.

HX tube rupture is often dealt with by bursting discs which require large diamter short vents with little in the way of bends. Your HX should be equipked with ESD valves to rapidly close and reduce flow from the broken tube and not

Other forms of two phase flow in vent lines, as BI says, are normally designed out as the impact, literally, of high veleocity liquid slugs or particles is such that it is not safe or economic to build it that way.

My motto: Learn something new every day

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

RE: Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

(OP)
LI & BI,
again, thanks for your interest. Based on above, conclusions could be:
1.- Avoid 2 phase flows whenever possible. There seems not to be a good method to work with 2 phase fluids
2.- In case one has a discharge which creates flashing, try the vent pipe to be as short as possible or try to separate the two phases as near as possible from the relief device or the point where the flashing may happen (knock out drum or similar)

RE: Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

Definitely a YES to both.
You really...ly...ly... don't want liquids flying out the vent, or possibly over-fueling, or extinguishing a flare.

Independent events are seldomly independent.

RE: Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

We have two phase safety valves and burst disks on our geothermal production lines. When calculating the trust force we have to consider all the possible combinations of fluid under the disk when the discharge happens. most of the time there is only steam at the top of the pipe and some times the line full of water.

Kevin

RE: Estimation of dynamic and stationary loads due to safety valves discharge with biphasic fluid

Why? Not draining condensates?

Independent events are seldomly independent.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources