starts per hour 5

[BRIS]

My field is civil/hydraulic engineering. I am looking for advice on the frequency of motor starts for pump sets.

I have a series of irrigation pump stations which will run continuously for 9 months per year. The pumps are fixed speed and will cycle on and off. I need to size the pump sumps to limit the cycle time. Normally I would allow no more than 8 starts per day but I am being pressed to reduce the sump sizes. The client's electrical engineer is suggesting as many as 10 starts per hour. The client also wants a long design life and minimum maintenance which would not seem to be compatible with 10 starts per hour.

Motor sizes vary from 20 kW (27 hp) to 600 kW (800 hp). Motors are air cooled. Ambient temperature may be 40 c (105 f).

Brian

 

[BRIS]

Further to above - will the provision of soft start/stop allow more frequent starting?

cheers Brian

 

[jraef]

Starts / hour ratings vary by manufacturewr. There are minimum ratings defined in NEMA Standard MG-1 but that only goes up to 250HP. Here is a link to the chart...

http://www.joliet-equipment.com/allowable_starts.htm

Above NEMA frames all bets are off, but each motor manufacturer should be able to tell you from the serial number. Be carefull, because I have run into large frame motors rated for 1 cold start plus one hot start PER DAY!!!

As to a soft starter increasing the starts / hour capability, this is widely debated. Some experts think not, since the reduction in current is coupled with an increase in acceleration time and the net effect is null. Some beleive that the increased time allows more dissipation through the cooling fans during the longer acceleration, as opposed to all the waste energy being pumped quickly into the motor. The concensus in this group over time seems to be that there is no appreciable net gain in starts/hour capability. I will say however that soft starters will definately make your entire pump last longer by reducing torque stresses that are mostly unnecessary in an irrigation application.

Quando Omni Flunkus Moritati

 

[mishlove]

Utilizing Electronic Soft Starting and/or VFD will increase allowable number of stops/start.

NEMA is best for small motors.

Larger motors consult motor manufacturers.

All subject to load WK^.

 

[electricpete]

VFD definitely makes the start easier.

To further jraef's comment... under condition of heavy load torque and low inertia, an improperly conceived soft start can make the start more severe. Other cases as discussed may have no effect or some improvement.

One thing to control in addition to the total number of starts per day or per hour (which limits long-term degradation) is the minimum off-time between starts. It is tabulatdd in the link above. If that limit is violated by starting a motor several times in quick succession without allowing cooling the motor may fail in short order from this short-term abuse.

 

[BRIS]

Thanks for the very helpful responses - .

I am pumping from a sump to a tank so fixed speed pumps are not a problem. My original design was for 3 duty and 1 standby pump at each station with each pump limited to only 8 starts per day. This give a fairly sedate design and low energy consumption. The client has complained that the sump and tank volumes are too large and he wants to increase the number of starts. He also wants to reduce the number of pumps from 3 to 2. (in fact the sump and tank costs account for less than 1% of the project cost but he has a "bee in his bonnet" on this issue). He is proposing up to 10 starts per hour per pump.

Reducing from 3 to 2 pumps will increase the daily power consumption. Also I would also assume that very frequent starts and cycle times will also greatly increase the power consumption?.

brian

 

[electricpete]

I think in general the energy associated with frequent starting is very small.

Wear and tear on the equipment is usually the big issue.

 

[GGOSS]

Hello BRIS,

jraef hit the nail squarely on its head. Only people who can advise you are the motor manufacturuers as the specifications will vary from one to the other. From your wording and you alias I would guess you are in Australia. If so, Nema standards won't mean much much to you, all the more reason to speak to the paople who build the motors.

It is doubtful that the addition of a soft starter would allow you to increase number of starts per hour as the I2t usually comes out about the same as that of DOL.

If you go the frequency/speed drive option then number of starts is not an issue becuase you would vary the speed to control the level in the holding tanks ie rather than use start/stop control.

Let us know if the pumps are submersibles. If so, there are other issues you will also need to concern yourself with.

Regards,
GGOSS

 

[BRIS]

Actually I am a Brit in Lebanon working as a technical assistant, on an Arab Funded project. The problem with these sort of assignments is that you have to be expert in everything or you have to rely on local support staff. I don't have a lot of confidence in my local support staff, hence I welcome the advice from this forum.

The pumps will be horizontal end suction. The pump stations will be located in remote areas. Maintenance standards will be low hence I need to keep everything as robust as possible.

I will follow your suggestions and will put my questions to a couple of potential pump and motor suppliers. Thanks for the advice - any additional advice is more than welcome.

brian

 

[GGOSS]

Hello BRIS,

My appology for accusing you of being an Aussie!

No problems with the pumps.

Soft starters and variable frequency drives may cause you some heartache if the cabinets into which they are to be installed are located outdoors. If this is the case, you'll probably be best served by electro-mechanical starting methods either full or reduced voltage and you will most certainly need to confirm max number of starts per hour with the motor manufacturer.

If this is to be a indoor installation with max ambient temp not exceeding say 35C, variable speed control would eliminate the need for frequent starting/stopping and will produce energy savings. This option therefore presents some real advantages worth further consideration.

Hope the above helps.

Regards,
GGOSS

 

[BRIS]

GGOSS

Thanks - I have been accused of worse than being an Aussie –but off hand. I just can't remember what it was

The cabinets are indoors - I expect ambient temperature will exceed 40 C and humidity will be high. If this is a problem with soft starters I will have to add a room with air conditioning.

regards brian

 

[jOmega]

Check with softstarter mfgr. There are limits as to how many starts/hour they can tolerate.

Because of this limitation... it is generally accepted that softstarters do not significantly increase the number of permissible starts per hour.

Conversation with BOTH motor mfgr (regardless of motor rating) and softstarter mfgr. is advised. Afterall, wouldn't you expect them to be the definitive authority on number of permissible starts per hour.

Consider that in sizing motors for pumping applications, mtr. mfgr. takes into account the starting duty and sizes motor accordingly. Motor capability must meet the application requirements.

Other factors include time-between-starts, which allow heat from starting to be dissipated and rms load while running as a percentage of rated.

So check with manufacturer is a good idea and the definitive word as to the allowable limits.

ElectricPete's suggestion of VFD is a good solution as it eliminates concern over thermal stress from frequent starts....and ..... an added benefit....reduces mechanical stress to pumps.... VFDs = kinder gentler start & stop.

Additionally, with VFDs, you may be able to improve overall efficiency of operation by using variable speed capability and removing energy wasting throttling valves.

jO

 

[BRIS]

When I started in the water industry 30 years ago I was instructed to avoid variable speed at all costs and I have stuck rigidly to this policy ever since. I am now wondering that maybe you electrical guys have made some progress over the last 50 years and perhaps VFD is now a viable option.

The responses are leading me to now consider using variable speed. This will greatly simplify the system hydraulic operation and the civil works.

Before I start pursuing manufacturers I would appreciate some further advice:

1) Is there likely to be a significant cost difference between soft start and stop and VFD.

2) Given that the pump stations are in remote areas (in the Bekka valley in Lebanon) and the standards of maintenance and maintenance skills are poor, is there any significant disadvantage in using VFD instead of fixed speed with soft start and stop.

Cheers (Don't call me a Ludite) Brian

 

[GGOSS]

Hello BRIS,

Pleased to learn this will be an indoor installation. You should note however when we refer to ambient temperature we are in fact referring to the temperature of the air flowing over the heatsink assembly of the soft starter or variable speed drive. As this is often higher than the external ambient, a review of equipment specifications/discussion with potential suppliers is suggested.

Variable speed will be significantly more expensive than start/stop control however the cost differential (and more) will be recovered over time through energy savings.

My only reason for not pushing the variable speed (or soft starter) technology harder here is because of your expressed concerns re the "standards of maintenance and maintenance skills". Your first experience needs to be a positive one, and unfortunately it would appear that cannot be guaranteed here.

Regards,
GGOSS

 

[BobPE]

bris:

I would have to agree with GGOSS that in your situation, maintenance is the key factor and VFD's will probably not fit in this duty very well.

I am a bit confused though...I don't see enough information in your posts to determine that there will be an energy savings through the use of VFD's....I agree with electricpete that they will save wear and tear on the pumps, but primarily on starting...operation across the line if the pump hydraulics are designed correctly will work fine...I think we need to know more about the discharge of the flow to determine potential energy savings of the VFD's.

BobPE

 

[BRIS]

Bob

I have long transmission lines supply water from a surface water impounding reservoir to irrigation tanks. The irrigation demand varies from zero in winter to a peak in June and then back to zero.

The pumps are basically booster pumps located at points on the pipeline. We are providing upstream break pressure tanks.

When I am pumping say 70% flow in August, with two fixed speed pumps, one will run continuously and the other will cycle on and off. Because of the long pipelines friction losses are high and when operating with two pumps I have maximum flow and maximum friction.

With VFD I would run the pumps continuously 24 hours per day and will therefore be pumping against a lower head and hence use less power.

As friction head is proportional to Q^2 and power is proportional to QH we use less power with 24 continuous operation rather than cycling between 50+ percent and 100 percent flow.

I would also save power if the pumps were to be installed as in line boosters, but we also have large variations in the supply head.

This is all good in theory but I now need to look closely at the system curves and find pumps that can operate efficiently over the large range of duty points.

Brian

 

[GGOSS]

Hello JOmega & BobPE,


JOmega; In your post of August 11 you said "Check with softstarter mfgr. There are limits as to how many starts/hour they can tolerate."

This is valid however you should note soft starters are capable of sustaining any duty cycle if they are selected correctly. For example we have many installed that are doing upwards of 300 operations per hour.

Prior to IEC60947-4-2 the user's choices were limited to selecting a product from a brochure. This is no longer the case. Product users are able to specify required duty and manufacturers/suppliers are obliged to offer a product that 'fits'.

BobPE; The original system proposed by BRIS was on of start/stop control. In addition you will note a substantial number of starts per hour were required due to 'pit' size.

By installing a Variable Speed Drive into this application, one would vary the speed of the motor to control fluid volume in lieu of start/stop control. The fact that the motor will be operated at below speed, will generate energy savings.

Regards,
GGOSS

Regards,
GGOSS

 

[electricpete]

3600 per hour = One start every 12 seconds.
I'm no expert but that sounds like a very poorly engineered system to me.

 

[electricpete]

Sorry, I shouldn't have used the term poorly engineered... I don't know the circumstances of the application.

I would think that most systems have some way to avoid that type of cycling... like an accumulator or some other adjustment to the mechanical system. But I guess there are a wide variety of applications.

One would think that even with a soft start the equipment will not last long.

 

[GGOSS]

Application was a palletizer.

Not withstanding that, the point was soft starters can be rated to any level of starting duty required. The 'weakest link' is generally the motor.

Regards,
GGOSS