Pumpback Loop to Storage Sphere

[RJB32482]

Hello,
I saw a pumping system that utilizes a pumpback loop to the storage sphere at all times for normal pentane. The pump transfers the pentane to an end source also, but always utilizes the line back to the sphere. What is the reason for this? Could it be:
1. always mixing the pentane (no stagnent pentane to heat up in the sphere in warm conditions).
2. heat from stopping the pump could cause pentane to vaporize causing cavitation in the pump.

Any other reasons or which of these are correct in your engineering opinion?

Thanks.

 

[Montemayor]

RJB32482:

Your #2 reason is just one of the main scope reasons. I doubt if #1 is part of the original scope. The reason I say this is that you identify the fluid as Pentane and I believe you are mistaken (or this is a typo error) - or the spherical tank is being used provisionally for storing this relatively “heavy” hydrocarbon. I say relatively “heavy” because even lighter hydrocarbons like butane and propane don’t “earn” the privilege of being stored in a sphere. Spherical tanks are very expensive storage units and are usually reserved for much lighter hydrocarbons.

If I were allowed to employ a physically strong (good pressure containing feature) tank like a sphere to store Pentane, I could easily justify a substantial Nitrogen blanket and the assurance that the Pentane in the sphere is always in the liquid state. Therefore, I wouldn’t be concerned with “stagnent pentane to heat up in the sphere in warm conditions”. Even if there were no N₂ blanket, the sphere can easily absorb the higher vapor pressures to maintain the Pentane in the saturated, liquid state.

What is probably one of the main reasons employed in the original scope is that the transfer pump’s total capacity is such that it requires flow control and there is a need also to avoid a dead-head condition on the discharge. A recycle back to the tank is always a sure and safe way to provide those needs.

 

[JJPellin]

We have a process that is set up as you desribe. We store a light Naphtha stream that is refered to as Natural Gasoline in some of our Butane spheres. We were forced to do this because the pipeline that ships in this product was sending it in a line that normally carries propane and lighter and we were getting environmental exceedances on our floating roof tanks with this product. In either case. we pump this product into the refinery with several high speed Sundyne pumps that are very picky about minimum flow. Since the flow demand in the unit varies considerably and to protect against dead-head condition if the control valve fails shut, we added a spill-back line to the sphere. In our case, this spill-back is controlled with a self contained regulating valve that opens automatically to maintain a total flow (flow to unit plus flow back to sphere) that is above the minimum flow requirement of the pump. Before we added the spill-back, we used to fail these pumps on low flow regularly.

 

[bingopin]

We have a storage sphere containing butene. The pumps are Sundyne type (70GPM, 300PSIG DP) As JJPellin mentioned, these pumps are extremely picky about minimum flow and are notoriously inefficient, ~13%. Drop below minimum flow levels for just a few seconds and the pumps gas up. Even in "normal" operation, you can feel the heat rise across the pump. Since the forward flow can vary slightly or stop momentarily, there is a minimum flow line back to the sphere to insure consistent pump operation.

A much better application for this service is a multistage vertical turbine pump. While more reliable and much more efficient, they require significantly more capital cost. Oh yea, they still need the minimum flow line back to the sphere.

 

[JJPellin]

We considered replacing these Sundyne pumps with vertical turbine pumps. But this posed a number of problems. In a typical Vertical Turbine pump, the mechanical seal is running at full discharge pressure. In our service this is over 300 psi. Pumping light product at these pressure presents another problem. Because of environmental requirements, we are required to use pressurize double seals on this product. At pressures above 300 psi, we cannot even consider using nitrogen blanets on the seal pots to provide the pressure. Nitrogen will desolve into the barrier fluid and the outer seal will fail from gassing up, dry running. I actually ended up asking for Sundyne pumps since there problem would be difficult to overcome. And if we regular our Sundynes close to BEP with the spill-back lines, we should expect decent reliability.