Hi all,

We have a 3.000 cm tank in which we keep a nitrogen pad. In order to maintain this pressure we use a blanketing valve, our desirable set point was 10 mbar because the pressure valve open at 30 mbar. One of our problems is that that when we are pumping product the flow meter of nitrogen reflex different ranges, for instance:

Product pumped (m3/h) Nitrogen flow (Nm3/h)
50 70
60 150
70 300
80 550
Until I know, the inflow of nitrogen is function of temperature, the pressure drop because of the evaporation/condensation phenomenon in the interface gas-liquid and directly proportional to the flow of pump. But what I see is that these relationship are different.

I would be pleased to received your advises.

Thanks, I have attached a brief file with a description.

Leticia

PD: Sorry for my English!

Leticia,

Thanks for reposting. First I'll repeat my previous comment here and then add some more to it.

There are a number of explanations possible. I am not suggesting that these are the answers but let me list what comes to mind.

Metering accuracy of the flow rates, both liquid and gas. You need to be reading scfh for the gas or if acfh, then measured at tank vapor space pressure, not gas supply pressures.

Venting of gas during pump off due to a combination of blanketing gas pressure control problems and pressure vent operation. (Some low pressure regulators have a pressure rise characteristic during flowing conditions to counteract droop.)

Also, do I see a sparge line into the tank? It is also connected to the flow meter so how is this being used?

First thanks,

I am not sure about if I have totally understand your answer. My product flow meter is tested and contrasted, It is ok because It is redundant. About the nitrogen flow meter I only have one, but the source of the nitrogen is a liquid nitrogen tank and the consumption of it is quite similar at the totalizer of nitrogen.

The product flow is pumped at 6 bar and the nitrogen flow meter is also at 6 bar but the flow is given Nm3/h in my scada system. What do you mean with scfh and acfh?

Thanks,

Leticia

Some flowmeters measure acfh (actual cubic feet per hour) and others are corrected to scfh (standard cubic feet per hour) or "standard conditions" rather than at compressed conditions.

What I meant was your liquid flow metered rate is always measuring actual=standard because the liquid is not compressed. This is not so with the gas measurement, unless you are measuring it at or near atmospheric (standard) conditions.

Well, I see what you mean, and I also think that could be my problem but there are two points:

- We use a thermal mass flow meter so It should be independent of the pressure.
- The relationship is not linear and following this principle It should be.
- Following the consumption of our liquid nitrogen’s tank I think this consumption could be true.

I find absolutely interesting your answer, please ask me for any other question that you think could be interesting.

Leticia

Actually, at this time, you have me stumped.

Anyone else out there reading this who can assist Leticia?

The only physical thing I can come up with is this: At low flowrates, the product in the tank will go into the vapor phase (Boil) at a rate such that it takes very little nitrogen to displace the liquid. As the flowrate increases, the product cannot vaporize quick enough (get heat from the outside) and it takes more nitrogen to replace the liquid.