Intermittent vs. Continuous Duty 2

[curtis26]

If the manufacture only can supply an intermittent duty pump curve for a reciprocating positive displacement pump, and you have a new application that requires continuous duty operation what general limitations and recommendation should be taken into consideration for use of the pump in the new application (i.e. pump speed, pressures, etc.)

The data sheet for the specific pump in question is attached.

 

[MikeHalloran]

You need to install two or more pumps, and provide a means to seamlesssly transfer operation from one to another without interrupting your process.



Mike Halloran
Pembroke Pines, FL, USA

 

[curtis26]

Mike,

You lost me on this one.???

Are us suggesting to line up quantity eight (8) pumps and run each of them for 3 hours a day (so that none of them exceed the three (3) hours per 24 hour period design definition of an intermittent duty pump)? This would not be an effective use of space, resources, and would not be practicle or economical solution.

An intermittent duty pump can be de-rated for continuous duty. The manufacture normally has a break horse power rating for the power end and a fluid cylinders pressure rating for the fluid end which they can go back to, to generate a continuous duty data sheet. The problem with the pump I referenced in the attachment is that it is no longer in production and the manufacture no longer has this design information. I was wondering if anyone had any insight on how to de-rate the pump pressures based on the intermittent duty ratings alone.

Also, I am aware of the API 674 Speed limitations, but do not know how the manufacturing speed limitations are rated or what is the justification behind faster speed ratings for the intermittent rating than for the continuous duty ratings for the same pump (or what speed de-rating should be considered for going from the intermittent duty to continuous duty operation)

 

[ash9144]

Why can your pump only run 3 hours a day? That makes no sense whatsoever. If your supplier is telling you this then he is an idiot and you need a new supplier.

 

[MikeHalloran]

Where did you get the 3/24?



Mike Halloran
Pembroke Pines, FL, USA

 

[curtis26]

Three (3) hours of operation during a 24 hour period is the definition of Intermittent Duty from the Hydraulic Institute of Standards. I have seen the same definition in a few other places. as well as a less conservative definition in the Cameron's Hydraulic book that defines intermittent duty as 4 hours per 24 hours period.

Yes, I agree that it is idiotic to think that just because the you only have a intermittent duty data sheet that you have to limit the pump to the that designation. Thus the question on who to de-rate the intermittent data sheet to arrive that the continuous duty limits.

 

[BigInch]

You could start by thinking about reducing the speed by 15 to 30%, similar to what you see here. This is based on motor service factors. You may want to vary your percentage of full load based on a speed correspondence to the forces, pressure, temperature or wear characteristics to whatever pump component you feel is most critical.

http://www.engineeringtoolbox.com/service-factor-d_735.html

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[micalbrch]

The only reasons I can see to have different performance data for intermittend and continuous operation is wear or oil temperature in the gearbox. The max. operating pressure must be the same unless any standard requires certain safety factors. Higher speed (rpm) causes more wear to plungers, plunger seals and valves and increases the gear temperature. But if you obey the max. speed limits of API 674 you are o.k.

 

[BigInch]

Could be, but is this an API pump???

Typically its the motor and generator specification that defines wholelistic limits to the service factor to load ratios, probably just because motors are more commonly used these days.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

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

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[curtis26]

Thanks BigInch and micalbrch!

FYI - API 674 is what pertains to this pump. I don't know of anyone that can comply 100% with API 674 so there aren't any "API pumps” when it comes to reciprocating plunger pumps, but we do specify API 674 speed limitation when the application require a degree of conservativeness.

 

[micalbrch]

Are you sure that nobody complies fully with API 674? This standard was specifically issued for reciprocating plunger and pistons pumps and I always thought that companies like National Oilwell and Halliburton fully comply with it.

 

[FMJalink]

curtis26,
I am afraid that is nearly impossible to determine, even nearly, the de-rate factors to move from what was applicable for the intermittent service to now a continuous service. If this pump was really sold as an API Std 674 pump for intermittent service, the API of that year would be applicable.

The API Std 674 of 1995 lists as main design criteria a minimum design life of 20 years and at least 3 years of uninterrupted operation. The first criterion concerns the total integrity of the pump system, and has probably been surpassed by several years in the intermittent service. The criterion of 3 years concerns mainly wear and tear parts.

If the intermittent service was specified as 3 hour per 24 hours period, but from your post it appears not that sure, you want now to use the pump for 8x the allowed operation time for the earlier allowed pumping service: pressure, speed and medium.

As already pointed out in posts above, several points can be loaded more for an intermittent service than for a continuous service e.g.:

• Wear parts will have in the 3/24h service have continuous service approximately minimally 3 x 3/24 = 3/8 year under equal loading (pressure and speed) for new pumps.

• Cyclic loaded parts will have a smaller lifetime for continuous operation than for intermittent operation, when equally loaded (pressure and speed). Exact difference can only be determined after full analysis of the pump or using the design data from the manufacturer.

• Drive system can be loaded more in intermittent service, than in continuous service.

• Lubricant as oil and grease can be loaded more in intermittent service, than in continuous service.

• Etc.

The datasheet of the pump shows what was allowed for the intermittent service. The pump did supply a specific pressure and have a specific speed during his life until now. If you compare these with the maxima detailed in the datasheet, you will get an idea of the margin that was included in the service until now. This can also give an idea of the still available lifetime for the new pressure and speed intended and the impact on wear parts.

As might be clear from these points you have to know the limiting points of your pump. From these points you could start to de-rate, but as you pump probably already surpassed the 20 years of lifetime, it is questionable for what you are de-rating.

 

[curtis26]

API 674 was developed for continuous duty pumps operating in a refinery under controlled conditions. If you get into any other application then you run into some catch 22's on API 674. This is why the manufactures always give an API 674 exceptions list whenever complete API 674 compliance is specified. I know that one of these catches has to do with fluid end material hardness. API 674 has a specification for the maximum allowable hardness for the Fluid Ends, but tp get a pump rated for more than about 3,500 PSI, then we have to accept the manufacture's deviation to API on the material hardness. The same is true on the hardness if you want a Fluid End material in anything other than Carbon Steel.