Restriction Orifice with High DP and % of liquid upstream 3

[Steve010]

I have a problem in a project i am working in now (still in the engineering phase).
The purge line for the flare is from the discharge of a compressor. so the design was to place a restriction orifice to drop the pressure from 1500 psi to 20 psi. I calculated the bore diameter of the RO and it was found to be 4 mm. The temperature downstream of the RO will drop to -35 C. The process engineer said that hydrates will form so decided to inject methanol upstream of the RO to prevent hydrate formation. The problem is that the required methanol injection for hydrate prevention made the stream a 2 phase flow (gas +2% methanol). I cannot place a drain hole because the drain hole should not exceed 1/10 of the RO bore which will yield a a drain hole diameter of 0.4 mm which is not practical. Also if i used a standard drain hole of 2 mm the flow will increase dramatically (because the drain hole diameter will be 1/2 the bore diameter). I am concerned about the accumulation of the liquid (methanol) at the RO upstream face. Note that this is the only source of purge for the flare and if it got plugged a flare flashback might occur.

Any ideas?

 

[Steve010]

What do you think ?

 

[georgeverghese]

You could install a weep hole at the bottom of the RO to prevent methanol from accumulating upstream ? Or better still, locate the RO on a vertical piping run.

Otherwise the only problem I see is with these low temps and brittle failure, and noise.

Min permissible RO size is 3mm in my experience - if you want to use a bigger RO, you could use a 2 step letdown - cheaper would be 2 ROs' in series. Use a thick plate RO

Are you sure this process engineer is willing to inject MeOH 24/7 into this line just to generate a tiny purge gas stream - purge gas is a critical utility, and now a pump and tank are required to make this work also - so the safeguard required here is an FSLL on the purge gas 1500psig feed line, which will be an integral orifice and dp cell - these will be required to account for partial blockage of the RO set due to loss of MeOH or insufficient MeOH. What is the integral orifice size available to suit this purge gas rate?

 

[Steve010]

Unlike orifice plates used in flow measurement , ROs are not manufactured with drain holes. Even if they are manufactured with a drain hole, the minimum drain hole size is 2 mm which is half the calculated bore diameter in my case (4 mm), this will increase the flow rate dramatically.
This is why i am asking about a pressure control loop with a fail open control valve (provided that the flow at valve fail open will not make the downstream pressure exceed the design limits of the flare knock out drum)

What do you think about this solution ?
Is it something reliable ?

 

[georgeverghese]

We cannnot use a PIC loop here - the flare pressure is not always at 20psi - it will most likely be just above atmsopheric if you have no flaring normally. The PCV would go wide open all the time trying to reach 20psi downstream.

Controls are used only when there are varying process conditions and you need to stabilise it. Here there is nothing varying - we have a fixed purge demand, we have a fixed 1500psig upstream, and downstream pressure, though it may be varying, is always in excess of critical pressure, so downstream pressure has no effect on purge flow. ( Critical flow pressure for typical hydrocarbon gases is approx 50% of upstream ie. flare pressure has to be some 750-800psig before it starts to affect flow). If upstream pressure were to vary, an FIC loop would stabilise purge flow.

If the drain holes are too big in comparison to the orifice dia, agree with your concern.

How about a single ball valve for isolation and a globe valve set in position for this flow - once the flow is set, remove the globe valve handle and lock open the ball valve?

What about the piping materials on this system - good for -35degC?

 

[Steve010]

if i understood you correctly, yes flow will not change until the downstream pressure becomes nearly half the upstream pressure (when it breaks the critical flow downstream pressure), but flow changes with valve coefficient Cv (which corresponds to valve opening) even at choked conditions. Each Cv has a different flow, which is the choked flow at this valve opening. I agree with you that this is a fixed demand.If a PIC loop is used the valve will be wide open in the beginning only until the flare header reaches 20 PSI (set point). It will be a very slow pressure control loop due to the large size of the flare header system (it would take long times to change the pressure of the flare header by varying the flow).

I am very happy regarding your statement "How about a single ball valve for isolation and a globe valve set in position for this flow - once the flow is set, remove the globe valve handle and lock open the ball valve?" because i proposed that to the process engineer, but he refused with no specific reasons. using a locked globe valve acts the same as the RO (fixed demand), and will solve the liquid/methanol accumulation problem so the whole problem will be solved. The process engineer asked me weather there is a problem with the PCV or not instead of proposing other solutions.

Regarding the temperature, site work has not started yet so specifying the appropriate pipe material will not be a problem.

 

[georgeverghese]

In that case, with an extra 2mm drain hole with gas going through it, flow will increase to (6/4)^2 = 2.25 x of required flow = 2.25 x 500 = 1125kg/hr - this is 1.1mmscfd - does seem like a lot of gas for purging.

You've got the extra methanol for this ? Has the process engineer calculated how much ?

If you 've got a design review / HAZOP with plant operations attending, they may complain - good luck.