line and pump hydraulics 4

elquesifilma · Jun 30, 2008

I need to move 60,000 barrels of water from point A (elevation 0 ft) to point B(elevation 100 ft). The distance is 5000 ft. The line is fixed at 8.8 inch internal diameter.

A tank at point A feeds my pump which will pump the liquid to a tank at point B. Using Hazen-Williams equation i calculate that i need 123 psig at the discharge of the pump to get to point B with 15 psig (to get on top of the tank assumed 15 psig would be sufficient).

My questions are:
1) What safety margins do engineers use for pump discharge pressures? Do I quote a 123 psig discharge pump or a 150 psig?
2) Is there any concerns if I enter the tank at 30 or 40 psig if I oversize the pump? What pressures should I hope to see at the inlet of the tank?
3) my system will vary from 30-60 thousand Barrels of water per day, should I have 2 pumps each to handle 30 barrels, or just 1 to handle the 60, what happens to it when only 30 are being pumped through it?
4) Can someone give me advice in how to control my system? The water going into tank A comes from a pump upstream of it which can be shut down if tank level gets too high.

Well, hope people find this problem interesting; I appreciate any comments, THANKS in advance!!

katmar · Jun 30, 2008

1) Safety Margin It is hard to give a generic number for a safety margin. You need to link it to the accuracy of your input values. When I see round numbers like 100 ft elevation, 5,000 ft length and 60,000 barrel/day I suspect they are not very accurate. Remember that pipes can corrode and get rougher in the long term. Generally pressure drop calcs can be within 10%, but you need to know what eventualities you are catering for.

2) Pressure at destination This is one of the more common misconceptions in hydraulic calculations. You cannot have 30 or 40 psi "left over" when you reach the destination. The pressure at the inlet of the destination tank is fixed by the pressure in the vessel and the head of liquid above the inlet. If the pump head is more than your calculated head for the desired flow, all that will happen is that the flow will increase until the head delivered by the pump matches the head consumed by the system. Find out how to plot a system curve on a pump curve to determine the operating point.

3) Turndown This depends on your control and maintenance decisions. A common policy in this sort of situation would be to install 3 off 30 MBBL/d pumps so that one can be repaired while you have two operating.

4) Control This is too wide a subject to give a comprehensive answer to here. Find a tame control engineer and discuss this in depth. There are many alternatives. At the simple end you can use "bang-bang" or "on-off" control. For example, if you used 2 off 30 MBBL pumps you could switch off both when the level in tank A gets down to 10%, start one pump when the level gets up to 30% and the second when it gets up to 60%, when the level goes below 30% stop one pump.

At the sophisticated end of possibilities you could install a modulating valve with a full PID controller.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

Artisi · Jun 30, 2008

is this a home work problem?

BigInch · Jun 30, 2008

The first configuration you study should be a pump-pipeline with a discharge pressure control valve at the end of the pipeline to tank.

15 psig is equivalent to a 35 ft height, presumedly to the HI-HI (pump shutdown) level of the tank.

Any excessive discharge pressure over 123 psig, will require a taller tank or a backpressure control valve to keep the discharge pressure at 123 or less.

Ask for the vendor to quote a pump with 285 ft head and a flowrate of 60,000 BPD @ BEP. When you decide on a basic pump model, impellers can be selected or adjusted to zero in on your target flowrates at best efficiency flow and head.

Note: that your velocity is about 10 fps, a bit high, so you need to check waterhammer surge pressures.

Note: I get less of a pressure drop using the Churchill equation, so if your equation is high, you'll need extra tank height or a discharge pressure control valve.

Of more concern is what happens at 30,000 BPD. Your flowing pressure loss is much less, so if you keep the 123 psig discharge pressure at the pump, you arrive at the tank with excessive pressure and will overflow the tank, if the pump is not shutdown. Alternatively, a discharge pressure control valve can adjust the discharge pressure accordingly to reach the top of the tank.

One or two pumps is sometimes an operating cost vs reliability decision. With 2 x the flow at max, it leads itself to a 2 pump configuration quite readily, but costs will generally be higher. Is there a situation where you should always be able to pump at least 30,000 BPD, when one pump is down? That's a good configuration decision point.

Another alternative may be one pump (or two) with either a control valve or a variable speed control, but that requires quite a bit more study, after you have reached a decision on the one or two pump issue, and if you can determine the real necessity to pump at different flowrates and, since it appears that you have a tank in the system, your tank will be able to fill and drain to accomodate changes in flowrates, so maybe you want to consider variable speed later.... or maybe not.

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