PD Pump Deadhead Pressure
PD Pump Deadhead Pressure
(OP)
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
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





RE: PD Pump Deadhead Pressure
I'll leave to others to explain more about the difference between centrifugals and PD pumps and why this is so.
RE: PD Pump Deadhead Pressure
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.
RE: PD Pump Deadhead Pressure
RE: PD Pump Deadhead Pressure
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"
RE: PD Pump Deadhead Pressure
RE: PD Pump Deadhead Pressure
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.
RE: PD Pump Deadhead Pressure
RE: PD Pump Deadhead Pressure
RE: PD Pump Deadhead Pressure
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.
RE: PD Pump Deadhead Pressure
Generally speaking, for a PD pump, the max discharge pressure is when something breaks, or the motor gets overloaded and dies.
RE: PD Pump Deadhead Pressure
RE: PD Pump Deadhead Pressure
RE: PD Pump Deadhead Pressure
Install a relief valve to protect all components.
Ted
RE: PD Pump Deadhead Pressure
Ted
RE: PD Pump Deadhead Pressure
Slow news day.
RE: PD Pump Deadhead Pressure
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
RE: PD Pump Deadhead Pressure
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.
RE: PD Pump Deadhead Pressure
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: PD Pump Deadhead Pressure
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
RE: PD Pump Deadhead Pressure
RE: PD Pump Deadhead Pressure
What is your motor´s FLA read at nameplate? and for what voltage?
RE: PD Pump Deadhead Pressure
lukin1977 - FLA = 21 amps. Voltage = 230 Single, 1 Phase, 60 Hz. RPM = 1745. Service Factor = 1.0. Full Load Torque = 15.1 ft-lbs. Start Config = Direct On Line. Breakdown Torque = 35 ft-lbs. Locked-Rotor Torque = 29 ft-lbs. Starting (in-rush) Current = 139 amps. NEMA Design Code = L
RE: PD Pump Deadhead Pressure
RE: PD Pump Deadhead Pressure
Given you normally run at less than 300 psi, I would set the trip setting on the breaker a bit closer to your max running current if this doesn't affect the max starting current ( 7 X FLC is high but not that bad so I would hope not).
not sure what you're going to do with this info now, but I wouldn't work on anything less than 1000 psi if you're going to work without a relief valve (bad idea IMHO) or set the relief at something a bit more reasonable like 400 so you don't get into a lock up / stall situation with your motor.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: PD Pump Deadhead Pressure
Ted
RE: PD Pump Deadhead Pressure
If the motor is already drawing 21A at 300psi and if FLA (nameplate) is 21 A, then 300psi is the maximum pressure you can get before you start to overload the motor