8 replies to this topic

[kamal888]

Sirs,

Appreciate any help on this - thanks. How can I maintain a positive pressure in the propylene tank so that pumping out propylene is possible to the last 5% or so.

Storage T -45C to 30C

thanks
kamal

[JLMONTREAL]

Sirs,

Appreciate any help on this - thanks. How can I maintain a positive pressure in the propylene tank so that pumping out propylene is possible to the last 5% or so.

Storage T -45C to 30C

thanks
kamal

Nitrogen purge could be applicable.

[Art Montemayor]

kamal:

In order for us to help you, we need your help first.

We need you to tell us:

• Is this in a design stage, or is it an operating unit?
• What type, size, and design of tank is it?
• What is the NPSH available to the pump – based on the liquid level being at the bottom of the tank?
• What type, size, and design of pump is planned or being used? What is its required NPSH?
• What are the conditions of the propylene in the tank? Saturated at -45 ^oC? Or saturated at 30 ^oC?
• What is the pump-out flow rate?

[kamal888]

kamal:

In order for us to help you, we need your help first.

We need you to tell us:

• Is this in a design stage, or is it an operating unit? - design stage Sir
• What type, size, and design of tank is it? Horizontal bullets 150#
• What is the NPSH available to the pump – based on the liquid level being at the bottom of the tank? calculated as 6.5 m at lowest vapor pressure (-45C)
• What type, size, and design of pump is planned or being used? What is its required NPSH? inline centrifugal NPSHR = 4.2 m
• What are the conditions of the propylene in the tank? Saturated at -45 ^oC? Or saturated at 30 ^oC? - always liquid saturated at op. T
• What is the pump-out flow rate? 120 m3/h

Thanks Art, my answers are in color next to your questions.

[kamal888]

Hi Art,

Thanks for looking up my questions - This is the second time I am using this forum and I seem to have done a mistake in reply. Here it is again.


Is this in a design stage, or is it an operating unit?
- design stage Sir

What type, size, and design of tank is it?
- Horizontal bullets 150#

What is the NPSH available to the pump
– based on the liquid level being at the bottom of the tank? calculated as 6.5 m at lowest vapor pressure (-45C)

What type, size, and design of pump is planned or being used? What is its required NPSH?
- inline centrifugal NPSHR = 4.2 m

What are the conditions of the propylene in the tank? Saturated at -45 oC? Or saturated at 30 oC? - - -always liquid saturated at op. T

What is the pump-out flow rate?
-120 m3/h

thanks
kamal

[Jack83]

Hi Kamal,

Tank dimensions and saturated temperatures are to be known. if you have cold liquid in the tank you can think of ambient pressure building vaporizers to maintain the tank pressure.As far as pumpable volume (5% in your case) is merely depends on the NPSH requirement of the pump.

[Art Montemayor]

kamal:

Now we have re-established that you are not handling Propylene at “-45C to 30C”. Saturated Propylene at 150 psia has a temperature of 20.5 ^oC while at 100 psia it has a temperature of 5.5 ^oC. Although this has nothing to do with your ability to pump out your 4 - 500 m3 tanks, I wanted to get all the facts down accurately - at the beginning.

The NPSH required by your selected pump is what will restrict you as to how low and how efficiently you can pump-out your tanks. I don’t know why you have selected an in-line vertical type of pump. At 150 m³/hr (approx. 500 gpm), this is a heavy and bulky pump that will require some head room and probably isn’t hermetic in design. It will probably require a foundation just as a conventional, horizontal model would. I don’t believe you would have any trouble in locating an acceptable centrifugal pump with an NPSHr of 4 to 5 feet. This would greatly aid and help in giving you the ability to pump out the tanks with ease and also help in getting better pump performance. I assume you are raising your bullet type tanks at least 6 feet from grade (bottom of tank to grade) and that they will be slightly sloped to favor liquid flow towards the end where the pump suction is located.

With proper height, good piping design, and a good pump you should have no problem pumping out the tanks dry. Are you using a “boot” under the tanks as your pump suction? This would raise the tanks up higher. I would try to avoid having to use an inert gas such as Nitrogen to increase the head pressure inside the tanks and assist your NPSHa. The nitrogen will dissolve in your propylene and this might cause problems downstream. If you select a good, sound mechanical design and a proper NPSHr in your pump, you should have no problems with the NPSHa in this application.

But you have to design those features in now, before freezing your design - especially the height of the tanks over grade.

[kamal888]

Hi Art,

Thanks for the convincing answer. The inline pump was selected by the pump vendor. I will have to ask him to consider the factors you mentioned. Nitrogen cannot be used as the propylene will be contaminated. Off spec quantities are measured in parts per billion. I am not proposing a boot as propylene is completely dry. This product is being loaded to rail cars from the new bullets and goes to a poly-propylene plant.

You are right I am not handling propylene at “-45C to 30C”, but that would be the range of storage temp all round the year considering.

My main concern was when the propylene level is about 5% in the tank there would be very less vapour pressure and the pump will not work. I was going to fix the low level trip at say 7 or 10% which is a further waste of my bullet space and I end up with a larger bullet.

Thanks again

[Art Montemayor]

Kammal:

The reason I raised the question about a boot is that a boot is one way to "trick" the unloading pump when you have a need to pump your storage tank dry.

The boot is nothing more than a "wide suction line" and is used as the suction source for the pump. A low-low trip switch is located in the lower section of the boot. As the liquid level reaches the bottom of the tank, it enters the boot where the level decreasing rate may increase rapidly - but not as rapidly as it would in a conventional vertical suction line. And if you size the diameter of the boot right, you get enough residence time for the level switch to trip out the pump with just a small amount of liquid left in the boot - but not in the tank. What we did in the "old" days was to manually drain the liquid in the boot off to another small vessel and proceed with venting only vapors left in the tank and on to preparing it for inspection, cleaning, and maintenance.

Using a horizontal pump under a bullet-type tank is quite conventional and fits in very easy with the use of a boot that has an elbow at the bottom that connects immediately to the suction line of the pump. This really minimizes the suction line losses and simplifies piping. I don't know if this suits your operation, but I thought you might be interested in how other operations have been installed where a complete pump-out is essential or desired.