Acceleration Head in AODD Pumps

[techky]

Hi Folks.
I need your advise.
Does using (Inlet stabilizer / surge suppressor / tranquilizer / pulsation dampener) on the suction side of the pump reduces the calculated acceleration head? If Yes, can we quantify it by using any equation / formulae?

 

[bimr]

Of course it will. The question is, what are you trying to accomplish.

 

[georgeverghese]

The formula for this may be found in the GPSA; also vaguely recall it is in the Hydraulic Institute Standards section on recip pumps.

Pls note a suction dampener will work well only if it it sized correctly (very often, believe it is not), and source vessel pressure is constant. It is better to reduce dependance on these devices as much as possible by selecting multiple head recip pumps (better than triplex at least) and/or using a NPSH booster pump. It is common occurence in industry that recip pumps lose capacity on volatile fluids application due this cause only.

 

[techky]

Hi Bimr
I have a very marginal difference between NPSHr Vs NPSHa after including the acceleration head loss.
NPSHr: 3m
NPSHa: 2.8m
After including Vapor pressure values as well.
That's why I am trying to see if Inlet stabilizer can reduce this acceleration head. If yes, how do we quantify it?


Hi georgeverghese
I am using a AODD Pump.
Can you please advise the HI std section you are referring to? Or the respective formula? Thank you

 

[georgeverghese]

Just found out AODD is a double diaphragm pump, and not a recip pump.

Have not seen pulsation dampeners being used for these.

These pumps are said to be "more tolerant" of vapor in feed - pls check with the pump vendor if your approach is necessary / adequate. How much have you have tweaked your calcs to make it look like NPSHa > NPSHr ? What are you pumping here ? If you have no room for pulsations in the feed, try a Moyno pump or some other non pulsating rotary pump.

 

[DubMac]

I don't think you'll ever win your fight using pulsation dampers (especially with a diaphragm pump). The suction side of a pump almost always wins its battles. Excess NPSHA is your best weapon. Do whatever you can to find some NPSHA. Can you physically lower the pump?

 

[moltenmetal]

Yes, a pulsation dampener can reduce the acceleration head requirement. But if you have inadequate linesize between the liquid source and the inlet of the pulse dampener, the pulse dampener won't fill adequately while the pump is on its discharge stroke. The pulse dampener is very much like your AODD pump, except that its "suction stroke" is slower and more gentle.

How much of that 3 ft NPSHR is acceleration head?

 

[bimr]

Agree with moltenmetal on this. The velocity head will be small component. You are more likely to have suction issues with a dampener.

 

[georgeverghese]

Agreed, if you have vapor in the feed to a AODD pump (due to NPSHr > (NPSHa - acceleration head loss)), it wont be able to develop the required discharge pressure.

Other rotary non pulsating pumps would include gear and screw pumps.

 

[techky]

Hi georgeverghese,
Pumping fluid is petrol / Pumping temp: 90 degC / Vapor pressure: 0.7Bara @ 90degc
Capacity: 5LPM
the pipe length equivalent is 60m / 1".
NPSHr: 3.2m (Inc of NPSHr + Acceleration head + Vapor pressure @ temp)
NPSHa: 2.8m


Hi Dubmac,
This is a suction head condition. So I presume no issues with dry priming.
The issue is with the overall distance from suction tank to pump flange (60m) & velocity head.
The inlet stabilizer supplier assures that the acceleration head can be reduced to zero by increasing the capacity (Meaning: use 2" instead of 1"). Thereby, reducing the pressure spikes by >83%.
I am having difficulty in coming to a logical conclusion here.


Hi Moltenmetal,
Noted on your point. Thanks.
- How about we use an Expander after the first elbow from Min. straight suction run & match the pipe size with inlet stabilizer? In that way, the velocity head should be slightly reduced due to bigger line size & inlet stabilizer getting filled.
- Or I use 1" Inlet stabilizer for 1" line size.
Please share your thoughts. Thank you

 

[bimr]

If all that you are concerned about is velocity head, the rule-of-thumb for selecting suction pipe size is to use one size larger than the pump suction connection.

 

[georgeverghese]

A gas charged suction side pulsation dampener wont work if this tank is open to atm.

Friction drop in 60m of say 22mm id pipe at 5 litres/min is low.

Raise the liquid height in source tank and / or decrease acceleration head loss.

 

[techky]

Hi bimr & georgeverghese
Thanks for your reply. The problem is that the system / piping cannot be changed. The pump needs to suit to this requirement.


Hi georgeverghese
May I know why is that the "A gas charged suction side pulsation dampener wont work if this tank is open to atmosphere". Since this was not informed by my Inlet stabilizer supplier. Moreover, this is a suction head.

 

[moltenmetal]

techky: obviously a gas-charged pulse dampener, which has a diaphragm with gas ABOVE atmospheric pressure, will not work on the suction of a pump if the suction pressure is BELOW atmospheric pressure. The gas will simply force the diaphragm all the way to its maximum extent (minimum damper volume) and it will cease to work as a pulsation dampener.

An empty chamber-type pulse dampener may still help you.

At the end of the day, does it matter? If you get some minor cavitation, you will lose some pump capacity. So what? Make the pump marginally larger. Petrol is a mixture without a single boiling point, so cavitation of petrol is much more gentle than it is for a pure component like water.

Why on earth is this "petrol" at 90 C? How often is it at 90 C- always, or is this a design condition that the unit will NEVER operate at? Cooling the suction line can help reduce the vapour pressure, if you really can't change the elevation head.

I'd be very worried about what happens when your AODD pump's diaphragm bursts and you introduce a large amount of compressed air into that 90 C petrol! The suction head would be the least of my worries frankly.

 

[techky]

moltenmetal: The temp is high as per the process requirement. The given info is operating condition. The pump is selected with larger capacity & therefore the speed is lesser comparatively. We are using a diaphragm leak detector to identify any mis-happenings.

So far I've not come across an empty chamber type pulsation dampener.

I am though not able to understand "if the suction pressure is BELOW atmospheric pressure". For a closed tank (Suction head) with no vent, the vapor pressure being high.

 

[moltenmetal]

In order to cavitate, your suction pressure at the very lowest pressure point in the pump suction. In an AODD pump, this point is probably within the suction check valve itself. That point must be less than or equal to the vapour pressure during the suction stroke, such that instead of pulling liquid into the pump to fill the area behind the diaphragm, instead this volume is instantaneously filled with vapour which then collapses back to liquid as the suction stroke completes. All this does is rob you of some capacity, and makes a little noise. It is unlikely to damage your pump.

The vapour pressure of your petrol is less than 1 atm at 90 C- it is not boiling inside the tank. Accordingly a pulsation dampener with a bladder/diaphragm in it which is pressurized to a pressure ABOVE atmospheric pressure, will hold the bladder/diaphragm against its suction limit stop- it will not vary in volume and act as a pulse dampener at all.

An empty chamber-type pulsation dampener is just like it sounds: it is a stand pipe- a capped piece of pipe connected to a tee, containing a bit of noncondensible vapour at start-up, and mounted as close as possible to the pump's suction. It has no diaphragm or bladder to separate the gas from the liquid. On each discharge stroke, it will fill completely with liquid, compressing the vapour in the headspace a bit. During the suction stroke, instead of drawing liquid from the tank through a long pipe with lots of frictional loss, the pump will draw down this standpipe a bit. It must be sized so that one stroke's worth of volume does not take the level in the standpipe below the NPSHR of the pump. You don't buy these- you make them out of pipe fittings. A small port at the top of the standpipe will permit it to be re-filled with nitrogen or another inert gas periodically, as the gas will eventually dissolve in the flowing liquid and leave the system Because your fluid has a substantial vapour pressure though, that's not such a big deal- it merely means you should make the standpipe a little taller so it can fill with vapour rather than requiring an inert gas fill.

 

[bimr]

Maybe this from Pipe Rules of Thumb will be helpful. "Flow-through dampeners. This design should be installed on a pump’s suction system when inlet pressure (NPSH) is very low and the fluid can contain entrained gases such as air, carbon dioxide, sulfur dioxide, etc. Flow-through units have significant advantages over the appendage design for a few reasons. First, the inlet and outlet diameters of the dampener are equal to the pipe diameter. Second, the in-line installation enables reduction of the inlet fluid velocity into the pump. In addition, the bladder response time is faster than an appendage dampener, due to the minimum distance between the shell inlet and pipe centerline. The flow-through design can also be used to improve the NPSHa of systems with low NPSH because it acts as a suction stabilizer."

http://www.pulsationdampeners.net/aodd-pump-dampener.php