10 replies to this topic

[FOTO]

I would like to know if there is a method that one can use to accurately size an orifice for pressurization purposes.
I believe this is quite complicated problem and I don't know how many engineers have come across similar issues and how they solved it.
Say for example, there is an infinite source of pressurized fluid that is to be used to fill a vessel originally empty to a certain much higher pressure, there is also a time range (minimum/maximum) to carry out this operation. How does one calculate the size of the orifice to be installed on the fluid inlet line to achieve this objective?

There is a depressurization tool in Hysys to perform the reverse of this process where you are relieving the vessel of its pressure either to a flare system or atmosphere, you can specify the time for this process and it would calculate the orifice bore size required for you. I don't know if the same tool can be used (if so, how) to perform this pressurization problem.

Looking forward to your responses.

Thanks

Francis

[Zauberberg]

Why would you need an orifice? Usually there are 3/4", 1", or 2" connections on the vessel where Nitrogen (or other inert) can be tied to. That's the common way of doing pressure pops in the plant. By introducing e.g. 2" valve and 2" pipe in the model, you can get good estimates of what the flow could be.

For estimating the time required to pressurize the vessel, you have to switch to Dynamics (if we are speaking about Hysys) and then watch how pressure rises by time. Once when you reach certain pressure, it will still continue to increase but at a significantly reduced rate - and that is exactly what happens in reality. You can create stripcharts and monitor any parameter of interest as a function of time.

I did that once when I was asked to estimate the time required to pressurize Mol Sieve vessel, and I remember that I have obtained results very close to those in reality.

[FOTO]

The reason for the orifice is to have a startup line to build pressure in the vessel gradually and in a controlled fashion so as not to cause damages to vessel internals due to sudden pressure changes. We have a system in which the internals are delicate and pressure surges during startup and shutdowns are to be avoided.

Thanks

[Zauberberg]

For pressurizing the circuit within the plant, normally there should be a startup procedure that explains how to introduce process feed into the system and build the pressure gradually - written in a perfectly clear manner. I have never seen that orifice is installed prior to startup for the purpose of vessel pressurization. How would you remove it when you complete the pressurization process?

[fallah]

For pressurizing the circuit within the plant, normally there should be a startup procedure that explains how to introduce process feed into the system and build the pressure gradually - written in a perfectly clear manner. I have never seen that orifice is installed prior to startup for the purpose of vessel pressurization. How would you remove it when you complete the pressurization process?

No need to remove that RO! It could be located in series (downstream of it) with an isolation valve in by pass line of main/higher size valve.

In start-up condition by pass valve is opened and main valve is closed,and after pressurization of downstream vessel, by pass valve would be closed and main valve opened.

In addition to avoiding damage of internals,such by pass line prevent any damage of main valve due to high torque may inserted to the valve because of high delta p during start up condition.

[Zauberberg]

I still think this is not a standard design/operating practice - not saying it hasn't been done anywhere.

Imagine that you have to do maintenance work on that particular valve that is bypassed during startup, while the plant is in normal operation: the bypass would never allow for the full flow (temporarily) due to the orifice installed for startup purpose, and you would have to shutdown the unit/plant. It doesn't appear good to me.

[fallah]

I still think this is not a standard design/operating practice - not saying it hasn't been done anywhere.

Imagine that you have to do maintenance work on that particular valve that is bypassed during startup, while the plant is in normal operation: the bypass would never allow for the full flow (temporarily) due to the orifice installed for startup purpose, and you would have to shutdown the unit/plant. It doesn't appear good to me.

You are right,but your statement doesn't relate to the subject.The by pass line we are discussing,is start up by pass not maintenance by pass (as you thought) and its design and sizing wouldn't be performed based on doing maintenance work on the main valve during normal operation.

Indeed,start up by pass or by pass Pressurization Line is commonly used in oil and gas facilities and its size is in the order of 2" including isolation valve and a RO/or throttling valve.

[FOTO]

Mr Fallah is right regarding the use of the RO in a startup dedicated line and seems to understand this issue. Have you been involved in the sizing of the orifice in the past or do you know how to go about doing it? This is usually a 1" or 2" line on a natural gas service and I would like to know the relevant approach to determine the size of the hole in the orifice plate to allow for the fluid to pressurize the system in a timely manner (not too fast, not too slow).

Any help would be appreciated.

[fallah]

I would like to know the relevant approach to determine the size of the hole in the orifice plate to allow for the fluid to pressurize the system in a timely manner (not too fast, not too slow).

-Assume a size for by pass line,e.g. 2".

-Find mass flowrate in by pass line (downstream of the RO) using a limited Mach number,let say,Ma.=0.75

-Size the RO using mentioned flowrate and maximum delta P (source pressure-initial pressure of downstream vessel).

Obviously,lesser assumed Ma. number leads to longer time of pressurizing.

[Himanshu Sharma]

I was looking forward to discuss this chemical engineering problem !!!

Pressurization and depressurization through orifice is a phenomenon experienced in cycle during regeneration of dryers working in lead-lag system.(for e.g. Refinery Units like isomerisation feed dryers, hydrogen dryers, Petro-FCC propylene dryers etc.).Time for Depressurization should be large enough to avoid excessive nozzle reinforcement and repressurization time in dryers is governed by flow rate that reaches fluidization limit.
While depressurization one can use HYSYS utility or this can be done manually by scratching your head .
Jokes apart, HCs have to be depressurized to flare having a backpressure of 0.1-1.7 kgcm2 g (LP Flare).Flow from Upstream always remain critical except for final few seconds(remember as far as Upstream P/Downstream P=2,flow remains choked).Check your system as adiabatic or add suitable heat transfer equation.
The next factor comes as density changes with the pressure falling down and you have to suitably substitute this term with something that can be integrated with in limits of depressurization. Once you finish this the job is over !
I made and excel spread sheet and compared with HYSYS ,my results were in the experimental range of error(in fact was more near to actual data available, no bragging but happens when you tailor made a solution to mock the process).

Repressurization is often more tricky, turn a few pages here and there at your Thermodynamics book and you will be better able to characterize the system. Check upstream/downstream pressure ratio; your flow may be choked for initial few minutes. Once the choked flow regime is over , you have to apply the orifice flow equation check in API )and have to suitably assume orifice coefficient .Calculate the flow and time.

I can hint from my experience in Regeneration Process of Dryers that orifice size for repressurization typically comes out to be less than area 1/16 square inch. Since my instrumentation deptt is not able to procure orifice size less than this, we specify this size as orifice for repressurization

If you think that its a lot of iterative calculation think of someone who does the same for a Hydrocracker depressurization where fluid enters supercritical state, life would appear easy!

[sheiko]

FOTO,

Just to better understand the issue, do you want to pressurize downstream the orifice until equalization to the upstream pressure (so that Pup=Pdown)?

Edited by sheiko, 26 March 2010 - 03:59 PM.