Shall we take NG blanketing pressure credit for Butane NPSH design? 3

[AB2006]

Have you any experience or do you know which plant using NG blanketing pressure credit for Butane pumps NPSH design.
Since I have been assigned to re-study the existing design that took NG blanketing pressure credit for pump NPSH design, I think using methane for blanketing is not a satisfactory option for NPSH credit.I do not think we can take credit for natural gas blanketing in the Butane sphere tank. Because methane dissolves in butane and raise the vapour pressure. Although solubility of methane in butane is low, a small quantity of methane is enough to raise the vapour pressure to the operating pressure. This can happen to the top layer of butane, however, the vapour pressure for butane in the bottom of the tank may still be lower ( essentialy depending on incoming butane composition) assuming there is no turbulance in the sphere. So we may be able to run the pump without running into NPSH problem as long as the butane level is higher, say 50% of the sphere. As the butane level comes down, methane solubility effect will be more pronounced and we will experience cavitation. Am I wrong?

Regards, AB2006

 

[CJKruger]

AB2006,

Different people have different views on this. My views are:

- If you design such a system, assume the liquid is at bubble point. No credit for blanketing gas.

- If you are troubleshooting an existing system, look at how much of the blanketing gas will dissolve. Assume it mixing completely, else it gets too complicated and situation dependent. A pump can take a small bit of vaporization before it "officially" cavitate.

 

[Montemayor]

AB2006:

According to the definition of Available NPSH (NPSHa), you are wrong.

According to the definition of what constitutes vapor pressure of a substance, you are also wrong.

I've often used blanketing gas to increase the NPSHa to my transfer pumps - both nitrogen and natural gas - and have never run into any trouble with the priming, pumping or operation of these pumps - dissolved gas or not dissolved. By referring to the definition of NPSHa and applying the corresponding and resulting equation, you should apply the credit for the system pressure existing above the liquid level in the tank - the blanketing gas pressure. Of course, if you want to be a belt and suspenders type of guy you are also free to disregard this credit and simply raise the liquid level or the tank to a higher level.

The vapor pressure of butane is a given - regardless of whether there is any natural gas dissolved in it or not. The gas in solution does not affect the value of the butane's vapor pressure. It might give you a different vapor pressure - that of a mixture; but that is a different situation. What you state: "a small quantity of methane is enough to raise the vapour pressure to the operating pressure" is not correct.

 

[djack77494]

AB2006,
The worst you could be accused of is being overly cautious if you adopt the conservative approach recommended by CJKruger. Personally, I would use this approach despite the advise given by our wise sage, Art Montemayor.

Art, I mainly agree with what you are saying. The vapor pressure of butane remains unchanged by blanketing. The presence of blanket gas will likely allow for easier priming of the pumps. However, over the long term, contact between the gas and liquid will tend towards an equilibrium condition that will tend toward the mixture vapor pressure approaching that of the blanket gas. In that case, you will have gained nothing (in NPSHa-NPSHr) over having no blanket gas. In addition, dissolving gases into a liquid can have a significant negative impact on pump performance, similar to cavitation.

This may sound a bit more negative than I intended. I will concede that the quantity of dissolved gas will likely be very small with dissimilar components. Also, the presence of blanketing gas will be no worst than if the pressure had not been artificially increased; it just may not be much better. In summary, I guess I'm more agreeing than disagreeing, but I am saying that I do not favor this approach for most NPSH problems.

 

[dcasto]

At one of our plants we were using propane to increase the NPSH and everything was fine, until the purchaser called us about high levels of propane in the butane. We switched to methane and the amount of methane that dissolved was within specifications.

 

[25362]

Thread798-129328.

 

[djack77494]

Thanks 25362. That's a great discussion of NPSH and how it relates to source pressure changes and use of "inert pressurization gases" in that thread. For me, this issue is fully resolved.