PD Pump Deadhead Pressure

[Krausen]

All,
I'm accustomed to reading centrifugal pump curves that show the deadhead (shutoff head) head/pressure represented by the pressure in the discharge line at zero flow against a closed discharge valve. However, I'm having trouble with a PD gear pump application and potential discharge line overpressure. The gear pump itself is rated to 3000 psig but will only have a 5 HP motor operating on 240 V 1 phase power. The system will only need 300 psig maximum, but will typically be lower around 150 to 200 psig range. In the event of a blocked discharge line, I am thinking this 5 HP motor will shutdown from overload or trip a breaker before pressuring up to anywhere near 3000 psig. Can anyone advise on what the max discharge pressure of this gear pump & motor combo could be?

This may be more of an electrical question. What I do know is the circuit is protected by a 40 amp breaker and the full load amps (FLA) of motor while operating at 300 psig & 19 gpm is only 21 amps. I believe the in-rush (starting) current is ~120 amps.

Thanks in advance

 

[DubMac]

Don't dare to use your motor overload protection as a safety shutdown. Positive displacement pumps are just that, they will positively displace fluid into discharge chamber until something gives. It must have a relief valve directly downstream of the discharge flange. Don't screw around and get cute here; just do it.

I'll leave to others to explain more about the difference between centrifugals and PD pumps and why this is so.

 

[Krausen]

DubMac-
Understood. A relief valve will be added here as a safeguard. Im just trying to better understand what the max discharge pressure could be in this scenario from blocked flow without any relief.

 

[georgeverghese]

99 times out of a 100, the overcurrent protection may kick in at some pressure depending on what the viscosity of this fluid is and the flow. If the OVR fails, the pump casing will most likely fail, or the shaft seals will blow at some pressure.

 

[itdepends]

Assuming some slippage the pump discharge pressure will rapidly increase until the work required from the flow and head exceeds the motor power capacity. Power draw is proportional to both flow and pressure so if you ignore slippage and pump efficiency- halve the flow and the pressure differential doubles, halve it again and the pressure doubles again.

Shooting the breeze here- I suppose under a dead head scenario the motor will draw full (locked rotor) current for as long as it takes to trip the breaker. Under this scenario the maximum pressure the pump could develop would be if there was zero slip you could use the torque rating of the motor (here I'd ask a sparky) to calculate the force applied to the PD element. Use the surface area of the PD implement to work out a force per unit area and then use that to calculate the pressure. It's a long time since I did physics and a no slip scenario isn't credible (some slip will reduce the pressure achieved).

In reality- follow the advice above. The pressure is likely to exceed your system rating (including the pump). PD pumps typically increase in pressure when dead headed until something gives. It's best if the part that gives is controlled- like a relief valve.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[DubMac]

The max discharge pressure will be equal the weakest point in the system. Period.

 

[Krausen]

itdepends - thanks for the response. The problem I have is the standard power equation breaks down for a PD pump deadheaded with zero flow. Same for the motor shaft torque equation when your motor is stalled (RPM = 0). Like you say, probably need to ask a Sparky what would happen. Obviously this isn't something you'd design or engineer. It's more a "what-if" question.

Dubmac - your blanket statement just rephrased the question. What is the weak point? gear pump unit is rated to 3000 psig, discharge piping is rated for 1480 psig.

 

[georgeverghese]

Check with the pump manufacturer what max continous pressure the shaft seals could operate at. Also, if this is a variable speed pump, there would be operating scenarios where the flow is low, discharge pressure has exceeded design pressure and current demand is below OVR setting.

 

[micalbrch]

The weak point in a PD pump system is usually never the pump (casing) but the discharge side pipe system (hopefully just a gasket). To use the motor load as a kind of protection will only work if the pressure rises slowly, for example because somebody slowly closes a discharge side shut-off valve. For a sudden rise the motor overload protection acts to slowly.

 

[LittleInch]

I had a look to see if there is an easy way and everything I looked at was working on the assumption that there was a pressure relief installed.

Sometimes this is internal to the pump, but most people put on an external one so they can verify it, test it etc.

Even a 5hp/3kW motor will have a significant amount of torque. You need to look up the motor data or test sheet and find out if a locked rotor test was done and what torque is supplies and what current it takes. If the LR current is less than your 40A then it won't turn it off until the motor starts smoking and burns....

The pump vendor might have a shut in pressure versus torque graph if you ask him nicely.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TenPenny]

"Can anyone advise on what the max discharge pressure of this gear pump & motor combo could be? "

Generally speaking, for a PD pump, the max discharge pressure is when something breaks, or the motor gets overloaded and dies.

 

[BigInch]

As I understand, starting and locked rotor motor torques are typically at least 150% of rated torque and may exceed that by another 50%.

 

[1gibson]

First of all, I'd argue that the term "dead head" can't even be applied to a PD pump.

 

[hydtools]

Pump pressure can be related to torque. Torque = pump displacement per rev times pressure divided by 2π. displacement in cu.in., pressure in psi., torque in lb.-in. If you know the motor stall torque, you can calculate zero flow pressure potential. I say potential because something will probably break before that pressure is reached.
Install a relief valve to protect all components.

Ted

 

[hydtools]

Also, motor rotor inertia stored energy will figure into component failure if flow is suddenly blocked.

Ted

 

[DubMac]

This entire post is like wondering what part of your body will break first if you don't pull the ripcord on your parachute.

Slow news day.

 

[MikeHalloran]

For reasons that should now be clear, some gear pumps include an internal relief valve, set to bypass the actual pump just short of a damaging pressure level. The intriguing questions is why they all don't have such a feature.

If your pump has such an internal relief valve, it may be possible to modify its setting to also protect the external plumbing.


Mike Halloran
Pembroke Pines, FL, USA

 

[Krausen]

All,
Thanks for your responses. While some may say this discussion is irrelevant, I'd argue it's interesting and worth understanding. I'd agree with 1gibson that the term "deadhead" should never be applied to a PD pump. "Deadhead" is a carryover term from centrifugals that does not apply to PDs. A PD pump cannot be "deadhead-ed" without destroying something (shaft seals, casing, discharge piping, motor, etc.).

I'd like to understand the motor side more if I could. Probably should be a completely different thread. In this case my locked-rotor & full-load currents are below the 40 amp breaker limit. My breakdown & in-rush currents are above the 40 amp breaker limit. The motor vendor did not provide the stall torque or current, but knowing this would probably explain what would happen in the event the discharge line was blocked, as hydtools pointed out.

 

[LittleInch]

Your breaker seems to be set quite high. In rush and starting current are allowed for within the motor starter unit for a limited duration.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hydtools]

How about a number exercise? You say you can deliver 19gpm at 300psi and draw 21amps at 240vac. That is 3.32 hydraulic hp drawing 6.77 electrical hp. That exceeds your motor rating of 5hp and the power conversion efficiency is only 49%. Something does not seem right with your system or measurements. Are you connected to 240vac or 120vac? Is the current draw figure correct? If amperage is directly related to torque, you will reach 300*40/21 = 571psi at 40amps if you slowly impede flow rather than a sudden shutoff. Pressure is directly related to torque.

If the system can maintain the 19gpm flow as pressure is increased, at 5hp, the motor rating, the hydraulic pressure would be 451psi, if your efficiency was 100% which it is not.

Hydraulic power at 3000psi and 19gpm is 33hp. Just more than 6 times your motor rating.

Ted