6 replies to this topic

[srin]

Hi All-
I am a Process Engineer and this is my first query on this forum. Thanks in advance.

Please help me in validating the formula used for calculating relief flowrate for blocked outlet scenario specified below. Also, I would like to get your views on assumption made and the query posted.


Background:
We have an ammonia vaporiser and the vaporiser outlet is provided with block valve. PSV is installed on the vaporiser and relief flowrate has to be calculated for this PSV for blocked outlet scenario.

Mass balance:
1. Ammonia vaporiser inlet
Mass flowrate: 4000 kg/h (Liquid ammonia)
Inlet pressure: 16 bar g

2. Ammonia vaporiser outlet
Mass flowrate: Liquid ammonia is 400 kg/h; Vapor ammonia is 3600 kg/h
Outlet pressure: 15.3 barg

The PSV on the vaporiser is set at 24 bar g

Relief flowrate calculation:
P1 - Max operating pressure which is 16 bar g
P2 - Relieving pressure (with 10% overpressure)
W1 - Vaporiser inlet feed rate (4000 kg/h)
W2 - Relief flowrate
W3- Vapor ammonia generated in vaporiser(3600 kg/h)
rho1 - density of ammonia vapor at P1
rho 2 - density of ammonia vapor at P2

Relief load, W2 = W3*rho2/rho1

Assumptions made:
Two-phase relief is not expected since the liq/gas ratio is less than 0.15

Query:
How do I provide additional relief flowrate for the liquid ammonia that comes out of vaporiser (400 kg/h) for this scenario?

srin

[latexman]

There is no information on the heating media, the capacity of the vaporizer, or on the control schemes. Knowledge of these areas are required to predict what will happen in the blocked outlet scenario.

"How do I provide additional relief flowrate for the liquid ammonia that comes out of vaporiser (400 kg/h) for this scenario?" Is the scenario that the vapor line and liquid line are blocked at the same time?

[Art Montemayor]

Srin:

I congratulate you on posting a concise and very well explained query. However, your query doesn’t furnish enough information. I think Latexman already hit upon some weaknesses in his response.

Please refer to my attached workbook where you will find a simplified sketch. I have assumed you have a kettle type of ammonia vaporizer for this application. I have also assumed you are installing your vaporizer directly in-line. In other words, you are not allowing your vaporizer to float with your storage tank or whatever vessel the liquid ammonia originates in. Usually I would attach the vaporizer to the source liquid vessel and take the vapor from off the vessel itself, allowing the vaporizer to make-up the vapor pressure in the vessel on pressure demand. But you can also install the vaporizer much as I have sketched it. It is all up to you. You have not explained how the vaporizer is connected or controlled. This is an important point because it bears on how you will have the vaporizer valved and shut down (or blocked off) and thus may set the scenario for the relief requirements on the vaporizer vessel.

I furnished the sketch in order to show how the vaporizer is important depending on the manner it is designed and installed. In the manner I show it, if you block only the vapor outlet (you haven’t stated anything about also blocking off the inlet, so we have to assume the inlet is left open) the setting of 24 barg on the PSV tells us it will never relieve. What will relieve is the source tank of the liquid ammonia since (as I stated before) the liquid ammonia inlet is not blocked off. The pressure built up within the vaporizer will back up into the source liquid tank. The system as you describe it is not logically correct.

What one normally does is to block off the liquid inlet to the vaporizer and allow the outlet vapor to feed the process pressure (which is always lower than the vapor’s pressure (I find it hard to believe that a vaporizer would have 0.7 bar pressure drop; however, I don’t know how you plan to control the vaporizer rate). The manner I show the in-line vaporizer, it could be controlled by vapor demand or heat medium flow rate.

How are you able to know that there will be 10% liquid entrainment in the overhead vaporized product? Why do you accept this? Or is this a contingency on your part to be conservative on the design? Normally, I would expect to produce a 100 % saturated vapor with this type of vaporizer. That is why I state that it is important to identify the type of vaporizer and the heating medium (as Latexman has pointed out). Also, the control scheme is important as you can see by the sketch and the explanation I've given above.

Where did you obtain the relief load equation that has the load proportional to the two liquid densities?

You have stated that you are designing for single-phase flow, so why are you concerned about how to handle the 400 lb/hr of liquid exiting with the vapor? If you have guessed at the liquid entrainment, then you can also guess that the total vapor flow rate relieved is saturated vapor. I fail to understand the justification for 2-phase flow unless you want it or need it that way.

Await you reply.
 Ammonia_Vaporizer.xls   926.5KB   410 downloads

[srin]

Hi Latexman & Art Montemayor-
Thanks for your quick response. My sincere apologies for not providing complete information. I have edited the attachment file with complete information.

Background:
Liquid ammonia is vaporized (90%) using ammonia vaporizer/ accumulator system. The vaporizer is shell & tube heat exchanger with ammonia on shell side (effective volume 1.5m3) and the accumulator is horizontal vessel of 2 m3 capacity. The outlet from vaporizer & accumulator is routed to super heater for complete vaporization (low pressure steam is used as heating medium). The accumulator is provided with PSV and the super heater outlet is provided with block valve; we are interested in calculating the relief load for this PSV for blocked outlet scenario caused by the block valve on super heater outlet pipeline.

Blocked outlet scenario:
Super heater outlet block valve is closed while the feed continues to enter the vaporizer; cooling water supply & steam supply continues to enter vaporizer & super heater respectively.

The PSV on the vaporizer is set at 24 bar g


Query:
1. Can I use the formula below for calculating the relief load for blocked outlet scenario?
This formula is taken From Perry (Chapter 26, Page 29)

Relief flow rate calculation:

P1 = Max operating pressure which is 16 bar g
P2 = Relieving pressure (with 10% overpressure)
W1 = Vaporiser inlet feed rate (4,000 kg/h)
W2 = Relief flowrate
W3 = Vapor ammonia generated in vaporiser(3,600 kg/h)
1 = density of ammonia vapor at P1
2 = density of ammonia vapor at P2

Relief load, W2 = W3*(2/1)

2. I assumed that two-phase relief is not expected since the liq/gas ratio is less than 0.15 (I found this criterion in one of our in-house manuals for process upset scenario. Please give your view on this; but, in contrary to this, another write-up says, two-phase scenario must be considered for similar system under consideration.

3. If the equation given in Question1 is correct, and if I use 3600 kg/h for W3 (vapor ammonia generated in vaporiser), how should I address the liquid flowrate that is accumulated inside the vaporiser (400 kg/h)?

Attached Files

•  Ammonia_Vaporizer.xls   935.5KB   210 downloads

[Art Montemayor]

Srin:

Thank you very much for the updated sketch you have supplied. Your process is seen much more accurately now – at least I now understand it better.

Please respond to the following:

1) Is this a preliminary design? Or is it an existing facility?
2) Why do you refer to the vaporizer output as being 2-phase? Do you know this as a certainty?
3) Your liquid ammonia feed to the vaporizer is at 16 barg & 43.3 oC. Are you really using “cooling water” as your heating medium? Why are you not using a hotter and dedicated heating medium?
4) Are you using a kettle type of vaporizer as I guessed? If so, why are you allowing liquid ammonia to enter the superheater? This doesn’t make sense if we presume your scope of work is to furnish slightly superheated ammonia vapor to process. Why not just dedicate the superheater to superheat? If your process is an existing one, has it been revised or changed using local design changes? If so, who did the changes and are there calculations to substantiate the design?
5) Who designed the temperature controls on the heating “cooling water” in the vaporizer coil?
6) What are the downstream pressure and temperature requirements for the vaporized ammonia?
7) Once again, where did you obtain the equation for W2?

It is very important to answer our questions because we are forced to presume a scope of work that may not apply or be totally erroneous. Please confirm your scope of work. If you don’t know what a scope of work is, tell us.

On preliminary analysis, I don’t see the justification for the arrangement you show. A submerged coil type of vaporizer (such as a kettle type) will produce essentially saturated vapor which will enter the vapor accumulator and reside there until it is consumed. This vapor, in turn, is fed to a superheater downstream, as required. The accumulator acts as a surge vessel for vapor demand and ensures a steady saturated pressure – which is what I would control (not the cooling water). But your answer to our questions and elaboration on your scope of work should explain what you are showing.

[srin]

Hi Art Montemayor-

Here are my feedback

1) Is this a preliminary design? Or is it an existing facility?
We are in detailed engineering phase; the process scheme is given by Process licensor

2) Why do you refer to the vaporizer output as being 2-phase? Do you know this as a certainty?
As per the process licensor; the superheater is designed for the two phase as per the mass balance given earlier

3) Your liquid ammonia feed to the vaporizer is at 16 barg & 43.3 oC. Are you really using “cooling water” as your heating medium? Why are you not using a hotter and dedicated heating medium?
Liquid ammonia enters at -33 deg C; the cooling water comes from one of the downstream coolers after taking heat; we want to take as much as heat from cooling water before returning the water to cooling water return header (energy conservation).


4) Are you using a kettle type of vaporizer as I guessed? If so, why are you allowing liquid ammonia to enter the superheater? This doesn’t make sense if we presume your scope of work is to furnish slightly superheated ammonia vapor to process. Why not just dedicate the superheater to superheat? If your process is an existing one, has it been revised or changed using local design changes? If so, who did the changes and are there calculations to substantiate the design?
Sorry for not updating the PFD; we are not using kettle type vaporiser; the vaporiser is shell & tube heat exchanger; this design could be due to the usage of available cooling water



5) Who designed the temperature controls on the heating “cooling water” in the vaporizer coil?
Process licensor

6) What are the downstream pressure and temperature requirements for the vaporized ammonia?
Temp: 120 deg C; Pressure: ~15 bar g

7) Once again, where did you obtain the equation for W2?
I took it from Perry; Chapter 26; Page 29;

It is very important to answer our questions because we are forced to presume a scope of work that may not apply or be totally erroneous. Please confirm your scope of work. If you don’t know what a scope of work is, tell us.

On preliminary analysis, I don’t see the justification for the arrangement you show. A submerged coil type of vaporizer (such as a kettle type) will produce essentially saturated vapor which will enter the vapor accumulator and reside there until it is consumed. This vapor, in turn, is fed to a superheater downstream, as required. The accumulator acts as a surge vessel for vapor demand and ensures a steady saturated pressure – which is what I would control (not the cooling water). But your answer to our questions and elaboration on your scope of work should explain what you are showing.

[Art Montemayor]

Srin:

With the Holidays over, I can now respond to your reply. I hope that Latexman also will have time to contribute any comments he has on this subject.

You have said you “would like to get your views on assumption made and the query posted”. My views are:

1. You are obviously dealing with a process licensor for an ammoniated product you haven’t divulged and this licensor is also designing the vaporization system upstream of the reactors. I am opposed to the proposed vaporization scheme for various reasons which I can’t go into at this point because it would lengthen the thread. However, suffice it to say that using a conventional heat exchanger instead of a vaporizer is starting on the wrong logical process step. I can easily design and furnish saturated, single-phase vapor ammonia in one simple step and superheat downstream with much better controls than that shown. You (or your company) have probably forfeited or relinquished the right to determine and select the preferred method of furnishing vapor ammonia to the process and may not have any choice but to accept what the licensor offers. In my opinion, the licensor is offering a cheap and inferior method. But that is my opinion – which doesn’t play any role in your project.
2. Your attempt to use cooling water as the warm medium in the vaporizer is a potential process bottle neck because of the tendency of the water to freeze in the vaporizer if constant, sufficient water flow is not ensured in the vaporizer. This is a problem that I prefer to eliminate by not using the cooling water. But that is my professional opinion, and it doesn’t count in your project.
3. I would, as I stated before, totally avoid the 2-phase vaporizer. I believe it presents nothing but future problems and is not logical for absolute, steady process controls.

If you are contracted with the licensor and you have no contractual rights to deciding what is best for you or what you prefer as an engineering design, then anything that we discuss here with respect to your process design is purely academic and has no bearing or importance in what will take place. That is why a Plant Design Basis as prepared by the Owner Company is so important in the early phases of any project. The moment you signed your license contract with the selected licensor you “bought the ranch”. Now, you must confront the reality of having to try to work with and operate whatever the licensor leaves you with - and that includes, I assume, the sizing of the relief valves on the vaporization system. Good Luck.