Diameter of suction pipe

[21121956]

Hello everybody:

In an existing above ground heavy fuel oil storage tank, the outlet pipe from the tank to the discharge pumps is 10 inches in diameter and 230 ft length.

For operational reasons, the pumping out flow rate has to be increased so, with the new pumps, it is required to have an outlet pipe from the tank of 20 inches (even though the calculations give a 16 inches diameter).

The questions are: is it technically allowable to leave a short piece of the existing 10" pipe and then weld to it the new pipe of 20" via a reducer? or is it unavoidable to drill a new hole of 20" in the tank?

Thanks in advance for your comments.

 

[Artisi]

Although not ideal, there is no reason why cannot keep a short section of 10" pipe as you have proposed, it just means that the head loss in this section of pipe plus the expansion piece will be marginally higher than it would be for a straight section of 20" pipe adding some additional head to the inlet losses.

 

[katmar]

By "technically allowable" do you mean
1. is it allowable in terms of the construction code, or
2. will it introduce excessive pressure drop?

If you question is per 1. above then I agree with Artisi - it should be possible to construct a piece of piping with this geometry within the code.

If your concern is the pressure drop then it is impossible to answer completely with the limited information given. However, you should note that a single 10" to 20" pipe reducer will likely cause a higher pressure drop than your entire 230 ft of 16" pipe. If pressure drop is a concern then you should have a special reducer fabricated with a very gentle taper - say a 15 or 20 degree enclosed angle.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[dickon17]

Find out if there is any 10 inch pipe inside the storage tank (before the pipe exits the tank). There may be a 90 degree bend and a suction bell. Assuming a significantly increased flow rate, you might want to check friction loss for all the remaining 10 inch pipe. You may have no choice but to replace it all with 16 or 20 inch.

 

[JasonG]

Excuse this dumb question from a civil engineer but this scenerio you describe has long perplexed me. I am more comfortable working with open channel flow. Perhaps you can explain this to me.

If I understand correctly, the 20" pipe has been chosen to increase the flow rate. Why wouldn't the short section of existing 10" pipe, if left in place, limit the discharge rate of the tank?

 

[dickon17]

When dealing with suction pipes, normally you only have atmospheric pressure available to create a sufficient flow rate to fill the cavities created in the pump. Even if this pump can create a 100% full vacuum, this only leaves about 14.7 psig to push the liquid from the tank, through the suction pipe and into the pump suction port. Having a larger diameter suction pipe reduces the friction losses, leaving more of the 14.7 psi available for increased flow.

A 10 inch suction pipe will restrict the outflow from the tank, which is the same thing as restricting the flow into the pump and therefore out of the pump.

 

[ccfowler]

21121956,

Your comment about 16" vs. 10" line sizing suggests that the flow rate is increasing by a factor on the order of 2.5. This implies much greater acceleration of the flow from the nearly stagnant state in the tank into the 10" related connection in the tank wall (or possibly suction bell fittings inside the tank). You may find it necessary to maintain a substantially higher level in the tank in order to permit stable operation if you do not upgrade the in-tank hardware.

If you have a suction bell arrangement in the tank, you may be able to modify that bell to provide more gentle acceleration of the flow while keeping the 10" piping inside the tank and through the tank wall. It would certainly be wise to follow katmar's recommendation for a special reducer fitting adjacent to the tank.

If there are some controlling conditions that make keeping the 10" connection at the tank wall necessary, but there is no 90 degree bend and suction bell near the tank bottom, then the addition of such facilities could make keeping the 10" wall penetration practical. The use of larger pipe inside the tank would be wise, but the reducer adjacent to the tank wall should be eccentric with the top being level. The taper of this reducer can be more severe than for the reducer outside the tank. In effect, you would be creating an approximation of a venturi with the throat at the tank wall.

 

[trashcanman]

If possible, I would move the pump closer to the tank. Have you calculated the NPSH to make sure it will work?

 

[21121956]

Hello everybody:

First of all, thanks to all of you for your valuables comments.

The fuel oil storage tanks I am talking about, have these dimensions:

Nominal diameter: 124 ft; Total height: 55 ft 8"; Nominal capacity: 120.000 BBL; fuel oil kinematics viscocity: 630 cSt @ 50º C.

The tanks have an outlet oil heater, bolted to the tank nozzle; from here on, the pipe diameter is 10" up to the reduction in the pump inlet.

At present, the mass flow delivered by one positive displacement screw pump is of 300 gallons per minute, through a suction pipe of 10" and 230 ft long and, a discharge pipe of 8" in diameter and 1665 ft long.

As stated in my first post, for operational reasons we need a pumping out of 900 gallons per minute. For this purpose and, in order to bring the fuel oil level in the tank up to its minimun (1,3 ft), we have made the appropiate calculations and, we have found that, for the new pumps station it is needed a suction pipe of at least 16" in diameter and 230 ft long which produce a suction pressure in the pump of around 2,6 PSI and a discharge pressure of around 82 PSI.

The new discharge pipe is of two sections: one from the outlet of the pump 233 ft and 8", and the other is 6" and 197 ft long to the receiver tank.

To dickon17 and ccfowler: the shell of the tank has a nozzle where it is bolted an outlet oil heater. There is no any pipe, nor elbow nor foot valve inside the tanks.

To trashcanman: for some security regulations (regarding to fire in areas with hazardous materials) we are not able to move the pumps station closer to the tanks.

Once again, thanks.

 

[dickon17]

Are you retaining the old suction heater? What is the pressure loss through the old/new heater? Will the flow always be 900 gpm? What is the NPSH required of the new pump? Are you keeping the old 300 gpm pump and buying a new 600 gpm pump? My calculations show the existing 10 inch suction line may still be suitable for 900 gpm. I must be doing something wrong. Is the oil temperature 50C as it leaves the suction heater? Are the storage tank and the pump station at the same level?

 

[21121956]

Hello everybody:

Hi dickon17, I have made a mistake, the suction pressure for the pumps is around -2,6 PSI instead of 2,6 as stated above.

The answers to your questions are as follow:

a. The tank outlet fuel oil heater (suction heater) is not an issue, we have an alternate solution for a heating source.

b. In normal operation it is supposed that two pumps will be running and the third will be in stand by but, it is true also that, several times in a week or in a month, the three pumps will be pumping out, in a reliability way.

c. Three brand new positive displacement internal gear pumps will be acquired. Each of 300 gpm, 100 PSI max. differential pressure and a minimum inlet pressure for the pump of -3,8 PSI.

d. According to our calculations, if we use the existing 10" suction pipe, the suction pressure at the inlet of the pump will be -3,68 PSI when the fuel oil level in the tank be around 13 ft from the bottom so, it would be impossible to take out that mass of fuel without damaging the pumps.

e. The temperature of the fuel leaving the suction heater is between 45º C and 50º C.

f. The fuel storage tanks and the pumps station are at the same level.

 

[Artisi]

Calculate the pressure drop to retain the 10"outlet/ increaser and the 16" pipe you propose - it will probably be ok.

 

[dickon17]

I have recalculated the pressure loss down 230 feet of 10 inch pipe at 900 gpm and 1,100 centistokes (45C). This comes to about 6 psi. You should be able to empty the tank, provided the pump NPSH required is no more than about 8 psi (this is quite likely). Get an exact NPSHR from the pump manufacturer. Also get them to check the suction conditions are suitable for their pump. They are selling you 3 pumps so they should not mind doing the calculation for you.

 

[itsmoked]

c. Three brand new positive displacement internal gear pumps will be acquired. Each of 300 gpm, 100 PSI max. differential pressure and a [red]minimum inlet pressure for the pump of -3,8 PSI.[/red]

Am I missing something dickon17?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dickon17]

itsmoked, yes you are missing something. You believe what the pump manufacturer tells you without asking "are you sure?" In my experience, a specific question always gets a more accurate answer. This is particularly true when there is an order for 3 pumps waiting to be placed. Why replace 230 feet of 10 inch pipe if not absolutely necessary? Asking this simple question takes almost no time.

 

[21121956]

Hello everybody:

For these tanks, the pipe for filling up has a diameter of 16" and goes directly bolted to the tank shell.

According to the normal operation of this terminal, from the tank that is being filled up, there is no any drain or delivery of fuel, and when the opposite action (emptying out) is achieved, there is no any supply of fuel to the tank.

For this reason, we have decided to install shut off valves properly located in these existing 16" pipes in order to install new branches of 16" pipes to feed the suction of the new pumps station.

For a pipe with 16" in diameter, when the fuel oil level in the tank reaches its lowest value, the suction pressure in the inlet nozzle of the pumps is around -0,147 PSI.

 

[BigInch]

Check the tank wall for the higher discharge forces.

http://virtualpipeline.spaces.msn.com

 

[25362]

And be sure that the roof vents are sized properly to cope with the "vacuum" produced by the increased withdrawal rate.

 

[21121956]

Hello everybody:

I highly appreciate your comments. Thanks for all.