Purging liquid line with N2 and meeting dP requirements

[tbaygen]

Hello all,
I am looking for some insight on more seasoned engineers in fluids on how we should tackle this problem. Today I have a distillation reciever (Tank 1) that we weekly will push over to a neutralizer (Tank 2). The fluid is dirty and will plug up lines if allowed to sit, so operations today will use a N2 purge to try and blow the line out clear each time. Our policy states that since it is possible toxic gas, we must meet at least 2 positive flow requirements or interlock the system to prevent backflow. 1 being flow, and the second a 5 psig differential between the utility (N2 and the dirty line).

The way its currently setup is the pushover line is first purged for 2 minutes with N2. Two XCV's open and an RO is installed to maintain backpressure on the N2 5 PSIG greater than the line pressure. The pushover line also has a control valve installed that opens to 50% during this inital purge, from what i can tell this is setup this way to prevent Tank 2 from overpressuring and blowing a RD (take a pressure drop across the valve before getting to Tank 2). This inital purge does not work properly because its hard to keep the 5 psig dP and not overpressurize tank 2.

Once the line is purged with the first step, next the dirty fluid is pushed over. Pressure above the liquid pushes the fluid to the neutralizer (tank 1 has 65 psig of pressure above it to tank 2). Once complete the N2 will then come on to try and push the liquid left over in the line to tank 2. This is where we have the most problems. When the N2 comes on we will constantly trip the 5 PSIG interlock and cannot establish any type of control of the pressure or line cleaning and still meet our policy for backflow.

Is this as simple as converting one of the two N2 XCV's into a pressure control valve, and getting rid of the orifice plate on the N2 line? Then putting in a real control valve (v-ball or something less likely to plug on the main dirty line). I cant fathom how an RO will work at all in this setup...

To complicate things further, we circulate NaOH in Tank 2 during all of this too.

I attached a basic picture to help illistrate whats going on.

 

[EmmanuelTop]

As long as you do not vent anything from the Tank #2, I do not see how can you avoid pressurization of the tank and choking of the purge flow. First, there is liquid volume displacement (from line to the tank), and then you possibly have some Nitrogen passing through the line and entering the tank. Both these factors contribute to pressure rise inside the Tank #2.

If I understood well, "tripping on 5 psig interlock" basically means zero or (almost) no flow - I suppose pressures in the N2 header and in the process line get equalized. So you have two problems: one is pressurization of TK #2 and the other one is low (or zero) flow through the purged line (with increasing pressure in TK #2 increasing the resistance to flow further).

You need to identify what is causing the purge flow to stop. If it is high pressure in TK #2, you need to find a way to manage this issue (perhaps an equalization line between TK #1 and TK #2 if there is no possibility to vent but this can also cause the flow to stall), or the problem is in excessive restriction for the liquid phase to pass between the tanks. Without identifying the root cause, it is hard to propose an optimum and correct solution.

Dejan IVANOVIC
Process Engineer, MSChE

 

[tbaygen]

The neutralizing tank (tank 2) is always vented with a controlled setpoint and pressure controller. The issue is during the pushover once the liquid line is pushed out (if we can get it to move before hitting the dP interlock, it can cause a high pressure rise (faster than the controller can respond) and interlock.

In terms of the dP, lets say N2 header pressure is 90 PSIG, if the pushover line (between 1 and 2) is 85 PSIG or higher, it will interlock and close everything.

I dont see how you can size an RO plate for gas flow, and liquid flow and make sure you meet a differntial too?

Tom

 

[tbaygen]

In addition the piping is definitely a mess. It looks like it was piece mealed together with varying sizes, restrictions and excessive bends/90s etc.

 

[EmmanuelTop]

What about adding a non-return valve in the N2 line? If you want to be double secured, use two dissimilar NRV's. This will prevent from the reverse flow (which is hard to imagine anyway, because in the worst case the two pressures can only be equal but not P(system) > P(N2)) if you are using Nitrogen which is normally at a higher pressure than the system pressure.

If you remove the orifice and the vent system (including relief valve, if any) is not sized for full Nitrogen flow, this is a hazardous situation I would definitely recommend to be avoided by all means.

Are you also saying that you have the problem only to get the liquid moving, and afterwards everything works OK?

Dejan IVANOVIC
Process Engineer, MSChE

 

[tbaygen]

Check valves are not allowed per our policy, assuming that is a non-return valve. We have trouble at all times with this nitrogen purge meeting the backflow policy no matter what status were in (initial purge, final purge), and i belive its because the combination between the orifice plate and downstream control valve change the outlet pressure on the orifice plate which casuses the flow to not be high enough and pressure drop high enough. The only thing that works correctly is the pushover portion which uses the pressure in tank 1 to push the haz material into tank 2 (n2 is off for that). After that is complete the line N2 purge is suppose to clear the line for next use but doesnt work.

What would be the correct control strategy? My gut would be:

1. Open the plug control valve 100% (this is the valve between tank 1 and 2 that is used to modulate flow of haz material).
1a. Maybe open Tank 2 vent valve 100% for N2 purge.
2. Open first N2 XCV
3. Convert Second N2 XCV to a pressure control valve and use the dP pressure to control its output (% open/close).
4. Allow 30 seconds for system to equilibrate then Interlock if 5psig dP is not met.

Thoughts?

 

[EmmanuelTop]

I have problems understanding the function of the control valve between the two tanks during purging. Why do you need it in the first place? Flow of N2 is determined by the N2 orifice size and pressure difference between the N2 header and the system. Unless there is something I am missing here, I would keep that valve fully open (and keep the valve bypass open, if it exists). If the N2 orifice is adequately sized not to pass more flow than the downstream system can handle, the valve is completely unnecessary.

Based on your description, the problem is in establishing as well as in maintaining flow of liquid without reaching the deltaP trip point. So it sounds reasonable to get rid of all the elements that create unnecessary pressure losses.

The second thing might be the actual volume of the system for which the DP is measured. If this volume equals just a few cubic feet of piping (or less), then any change of pressure is going to be very rapid and there would be no time for the system to react. I have seen similar effects happening between two control valves located close to each other - the surge volume available for absorbing fluctuations of pressure is very small and hence only a few miliseconds of blocked N2 flow will cause dP trip. This would normally not happen if the controlled volume was larger enough. Then you are in a really tough situation.

I would also try to introduce N2 during the gravity flow of liquid from T1 to T2. Moving a dead/stagnant volume of liquid requires additional force (it requires more energy to accelerate a body than to keep it in motion).





Dejan IVANOVIC
Process Engineer, MSChE

 

[EmmanuelTop]

Also try to decrease the DP trip set point. 1-2 psi should be more than sufficient.

Look for effective ways to increase the controlled volume. Also tap the N2 line at the highest possible elevation point. This will minimize the theoretical possibility of liquid ingress/backflow. Liquid can not normally climb uphill.

Dejan IVANOVIC
Process Engineer, MSChE

 

[tbaygen]

EmmanuelTop, thanks for the insight.

"I have problems understanding the function of the control valve between the two tanks during purging. Why do you need it in the first place?"

The control valve between tank 1 and tank 2 is only used for the hazardous material move, as we control how quickly it enters tank 2 to allow tank 2 time to neutralize the haz material and not build up heat (caustic reaction that generates heat).

"Flow of N2 is determined by the N2 orifice size and pressure difference between the N2 header and the system. Unless there is something I am missing here, I would keep that valve fully open (and keep the valve bypass open, if it exists). If the N2 orifice is adequately sized not to pass more flow than the downstream system can handle, the valve is completely unnecessary."

You are correct that the orifice should be sized for the correct amount of N2 flow and pressure drop, but the problem exists, that Tank 2 is higher elevation than tank 1, and thus a liquid static height sits ontop of the discharge of the orifice plate + whatever tank 2 pressure is sitting at. When the N2 flow comes on (P1), the dP across the orifice plate is always changing since you are moving the liquid head off the discharge side of the orifice to a point in which there is no more liquid and you are free flowing N2 gas (P2). This changes the dP across the plate namely P2, the flow and velocity of course, causing all my problems I think.

"Based on your description, the problem is in establishing as well as in maintaining flow of liquid without reaching the deltaP trip point. So it sounds reasonable to get rid of all the elements that create unnecessary pressure losses."

Agree 100%, however that is a large investment and is the last solution.

"I would also try to introduce N2 during the gravity flow of liquid from T1 to T2. Moving a dead/stagnant volume of liquid requires additional force (it requires more energy to accelerate a body than to keep it in motion)."

Cant do a N2 Purge when Tank 1 valve is open, its connected to a distillation column running at equilibirum pressure/temp.

"Also try to decrease the DP trip set point. 1-2 psi should be more than sufficient. Look for effective ways to increase the controlled volume. Also tap the N2 line at the highest possible elevation point. This will minimize the theoretical possibility of liquid ingress/backflow. Liquid can not normally climb uphill. "

Our requirement is 5 PSIG vs system pressure (the haz/lethal side). If i tap the N2 higher i will leave all the liquid in the transfer line still which is counter intuitive of what the system is trying to do.

Very frustrating indeed.
Tom

 

[EmmanuelTop]

With so many immanent hazards and associated limitations imposed on the process and safety controls, and yet the system is still operated in the "rope-and-stick" fashion and no investments for upgrading the system are acceptable because of the cost. You can't get a Ferrari with 100$ in your pocket. Or make a small Toyota go 200 miles per hour. I fully understand your frustration.

Something that does not work, should be redesigned and upgraded or replaced accordingly.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Would agree with suggestion to remove the RO and add a control valve - but this control valve should be a DPCV (not a PCV) set at a few psi dp higher than trip setting - you could set up a DPIC with field control valve or alternatively a self contained DPCV

Would your safety requirements permit you to remove the second XCV given this is the interface between lethal gas and N2 ? - maybe not?

This control valve sitting in between the 2 tanks may need to be automated to operate on tank 2 pressure control also - if tank 2 pressure goes high after the liquid plug is pushed out into T2, there may be a high pressure gas plug behind it - and this control valve will then need to quickly choke down to keep T2 pressure at a safe value.

Will this work ?

 

[tbaygen]

georgeverghese,
Thats a great suggestion on using the control valve for tank 2's pressure as well. I hadnt thought of that. One of my worries also was the slug of high pressure N2 following the liquid and how quickly the controller could adjust Tank 2 before tripping, or losing a RD.

I would have to have 2 block valves to be compliant between the N2 and lethal side. I think they would allow an XCV/DPCV with limits as a block valve though.

Thanks for the great suggestions.

 

[georgeverghese]

If your N2 supply pressure is constant at 90psig, then a PIC - PCV(as you had suggested earlier) would be the same as a DPIC - DPCV. Moreover the PIC / PCV would allow added flexibility to control downstream pressure.

If you convert the CV between T1 and T2 to act on high pressure control at T2, this may need to be configured as a separate PIC, given that the PID constants on this new PIC may need to allow for fast action on this CV - the CV trim style should allow for a large change in valve Cv for a small change in valve stroke.

 

[georgeverghese]

Though this is not the topic of this discussion, some other observations:

a) For lethal and toxic services, automatic isolation with utilities is typically fully automated double block and bleed - in this case the bleed is not automated.
b) To avoid confusion, the double block XCVs' should be tagged as XSV - their logics in PLC should be write access limited also.