Closed Loop Systems - Pressure and Head Correlation Help

[mobgma]

New to the industry and was looking for some help from any experts to clear up some confusion. Sorry about the formatting, copy+paste didn't work as I had anticipated.

Client RFQ:

Type: Centrifugal Slurry Pump - Lined, Intermittent duty (4-6 hours daily usage Monday to Friday)
Discharge size: 2 in. and 4 in. (2 pumps)
Discharge pressure: 300 psig

Motor
Type: Electric, 230 VAC/3-phase/60 Hz, TEFC.
Control: VFD (wired to motor at factory).
Duty: Move sand slurry around a loop.
A 3-m section of the loop is a porous filter pipe.
The slurry flows axially through the pipe while water oozes through the pores or slots in the pipe wall.
Pore diameter range: 40 to 200 µm.
Nominal slot width: 250 µm.

Flow
Normal velocity: 5.0 m/s (16.4ft/s)(Approx 150USGPM on 2” - 650USGPM on 4”)
Maximum velocity: 6.5 m/s (21.3ft/s)(Approx 200USGPM on 2” – 850USGPM on 4”)


Slurry
Maximum solids content: 70 wt %
Maximum slurry S.G.: 1.8
Apparent viscosity: 0.355 Pa.s (355 cP)
Temperature range: 20° - 60°C
Abrasiveness: High
Maximum particle size: 2 mm
Particle d50: 180 µm
Solids S.G.: 2.65

Piping
Nominal I.D.: 2 in. and 4 in.
Format: Hose
Total length: 18.3 m (60 ft)
Elevation of discharge over suction: 2.44 m (8 ft)

I am confused about the correlation between pressure and head. I see the differential head is about 10 feet lets say. However if he requires 300 psig discharge pressure, does that mean he will need the pump to produce (300psi x 2.31=693 Feet) + 10 Feet Differential Head = 703 Feet? I am not accounting for viscosity or S.G. either as well as any other possible items he may have down line.

Also wouldn't a slurry pump have difficulties generating this kind of head? In this small closed loop I am unsure if the pump which is basically pumping into itself can handle 300 psi back on its inlet.

Any help would be muchly appreciated. If I am unclear let me know and I will try to elaborate. As I indicated I am new to the industry so my knowledge is limited.

Thanks

 

[BigInch]

No, you are calculating head backwards. Pressure is divided by SG. A heavier liquid obviously takes less head (height) to make any given pressure than a light liquid would.

That is a very poor description by the customer of what he wants. He is obviously trying to do this without the benefit of an engineer writing up a proper specification. Your problem is that the customer should be the one that specifies the required head at each flowrate. It seems that he wants you do do it for him, probably for free.

If it was not a closed loop system, it would be customary to use the 300 psi to calculate necessary head. Since it is a closed loop system, he might not need a lot of head, but it is not clear, as you have apparently calculated the head loss in the piping and filter system at 10 feet. "I am not accounting for viscosity or S.G. either as well as any other possible items he may have down line." is taking a big chance. That is why the customer should specify exactly what he needs. I think 10 feet is very low, but maybe it is correct. I have no way to know. If he only needs 10 feet differential head, it makes me wonder why he needs 300 psi discharge pressure. Maybe he wants to do some pumping across a valve cutting the pressure by 300 psi? We don't know.

The customer should specify both the required head and the required flowrate at the prime operating condition for each pump. I assume that would mean a normal flowrate of 150 gpm for the 2" discharge pump and 650 gpm for the 4" discharge pump.

He should also specify both head and flowrate for any other operating conditions he wants, so he should also provide the required head at the maximum flowrates too.

He has apparently only specified a 300 psig discharge pressure. The problem is that you don't know the suction pressure, so you can't accurately calculate differential head needed by the pump. In that case, I would assume the suction pressure is near zero psig and offer a differential head calculated by using that discharge pressure.

Tell him you have calculated the required differential head by assuming that the discharge pressure he mentions is the Pump Differential Pressure and is equal to
300 psi * 144 in2/ft2 / 62.4 pcf / 1.8 = 385 feet
, but it might not be conservative to do so. You should verify the MINIMUM suction pressure with the customer at the highest flowrate.

Now you must determine what flowrate he needs that head for. It seems that he needs that same head for both normal and maximum flowrates. Giving him pump that would supply the same head at those widely differing flowrates is not going to be an efficient use of equipment.

Assuming he needs that 300 psig at both flowrates, you will have to supply a pump that will give 385 feet of head at both 150 and 650 gpm for the 2" and 200 to 850 for the 4".

That's 114 hydraulic horsepower at 650 gpm, but only 1.5 horsepower at 150 gpm

That's 35 hydraulic horsepower at 200 gpm, but 2700 horsepower at 850 gpm

A VFD seems like it would not supply what is needed by this system.

In my opinion, it could be very likely that your customer would be much better off with a pump for normal flowarates and a pump for maximum flowrates.


"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[mobgma]

Thanks for your reply. I have him elaborating on the suction conditions.

 

[BigInch]

Hope that's not all he's thinking about.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[Artisi]

Without spending too much time trying to analyse your pump application my intial comment would be, I doubt this is a centrifugal pump appliction.

 

[mobgma]

This is what I received back from the client. See below.

"I requested a check on the length of the loop we’re using for crossflow filtration. It came back at 60.5 ft. This would be the length for both pumps. Please note that the elevation change of 8ft was measured from the ground, rather than from the pump discharge. I figured it would give us a safety margin. Let me know if you disagree. Again, this elevation change is applicable to both pumps.

In the feed tank, the slurry level will be 6 ft above the ground. I hope this will provide sufficient suction head to either pump. If not or if doubtful, be sure to let me know. Also, we’re looking for a discharge pressure of 300 psig from each pump.

I suspect you might be wondering about these two pumps. Let me clarify. My understanding is that the basic testing will be done using the pump with the 2” discharge, as we have been doing to this point. The pump with the 4” discharge will be employed to check our scaled-up values."

Big Inch can you comment on the above?

 

[BigInch]

Why is the 300 psi needed?

I think you need to read again what I already said above.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[Artisi]

I think the whole idea needs looking at, 70 wt% solids at 300 psi etc - this is not a centrifugal pump application in a 2" or 4" pump.