Acceptable (min/max) runtimes for sewage pumps? 4

[HnH-Modeler]

Hello,

I am analyzing sewage/wastewater pump run and cycle times for a stormwater pumping station design. I've found guidance on min. pump run times (I see many sources say at least 1 minute), and total min cycle times of 10 mins, but I'm wondering if anyone one has any insight on maximum sewage/wastewater pump run and cycle times? Due to our situation with head/storage capacity we are limited on what pumps we can use, and I'm finding the ones that fit are running for long (10-30 minute run times with 20-min to 1-hr cycle times, with a few storms in long term record producing 1 hr run times). I just haven't found much guidance on whether sewage pumps can be run for that long.

Any insight or advice much appreciated.

 

[TugboatEng]

Pumps are usually rated for continuous operation. It's frequent starts that can lead to overheating and damage. Long run times with long cycle times is ideal.

 

[bimr]

You need to contact the pump manufacturers and get a specific recommendation on the actual pump and motor size that you are specifying (which you haven't posted). You can't make generalizations of pump run times for all pump types as it is somewhat specific to the particular application. The 1 minute minimum pump run time that you mentioned seems to be for an unusual situation. Pumps are designed to operate continuously and it is the excessive starts/stops that reduces pump life.

Garr Jones' Pump Design Station book has the following paragraphs.

"The active storage (between HWL and LWL) in pumping stations for constant speed pump motors must be sufficient to limit the number of starts and extend resting periods so as to avoid the overheating and over stressing of the motors that reduces their life. The tendency is to design oversized wet wells, but applying an excessive safety factor results in infrequent pump starts during the nighttime minimum flow rate, and the long storage times promote stagnant, anaerobic conditions that result in odors and corrosion. Some of the advice in the literature on allowable frequency of motor starts is based on the use of standard motors and is too cautious. Good judgment is needed to optimize life-cycle costs, so consult pump manufacturers rather than motor manufacturers, many of whom do not understand the overall picture and the need for objective compromise between motor and starter life, size and cost of sump, and number of pumping units necessary to achieve lowest life-cycle costs. Standard motors of moderate size with full-voltage starters can withstand about four starts per hour with no effect on motor life. Special motors and/or starter systems may be needed to increase starting frequency to at least 6 starts per hour for dry pit motors and 12 starts per hour or more for submerged motors."

"When designing a pumping system, the decision needs to be made as to the maximum number of starts per hour required (i.e., whether to stop the pump or let it continue to run at no load). To make this
decision, a number of other factors must be considered:
• Type of motor
• Motor horsepower
• Motor full-load speed
• Power utility restrictions on inrush current or frequency of starts
• Power utility demand (kVA) charges
• Reductions in expected motor life due to the frequent starting.

The frequency of motor starts is given by NEMA in both MG-I and MG-10. The two publications approach permissible frequency of motor starts differently. MG-I provides guidance on the number of successive starts (i.e., two starts from ambient or one start from rated load operating temperature). MG-10 provides guidance on the repetitive start-run-rest cycles applicable to pumping station operation. Because pumps are low-inertia machines, and because there must be a balance between first cost and service life, it seems reasonable to use about two-thirds of the starting frequency calculated from MG-10. Because pumps are low-inertia machines, and because there must be a balance between first cost and service life, it seems reasonable to use about two-thirds of the starting frequency calculated from MG-10. The allowable frequency of motor starts is, however, far more complicated than is indicated by such a calculation, because (1) an under-loaded motor may be started more frequently than a fully loaded one; (2) the required frequency of starts in a pumping station with constant-speed motors is based on the assumption that inflow to the wet well is exactly one-half of the pump capacity, whereas half of the time, the required starting frequency is nearly 40% less (see Figure 12-26); (3) the specified severe ambient temperature conditions may persist for only short periods of time; and (4) motors can be custom designed and built to withstand many more frequent starts than standard motors can. The most important factor relating to the frequency of pump starts is whether (for multiple pumps in a station) automatic sequencing is used. Some engineers do not favor automatic sequencers and prefer manual selection of lead and follow pumps for better control of pump wear. Programmable logic controllers (PLCs) can be programmed to alternate the lead pump at every cycle reliably, and the reliability can be increased by automatic self-testing and switching to a backup PLC if a malfunction occurs."

"Motors 224 kW (300 hp) and larger are usually custom engineered to whatever requirements are specified. Hence, large motors can be specified to provide whatever frequency of starts is needed for the
pumping station. However, if the frequency of starts is not specified, the motor will be designed to provide the same frequency of starts as small and medium sized motors (i.e., standard).

Submersible motors, cooled by the pumped liquid, can usually withstand very frequent starts—sometimes much more frequently than 20 starts per hour. For such motors, it is generally the starter (not the motor) that limits the frequency of starts.

Because objective decisions concerning allowable frequency of motor starts must be based on such a myriad of site-specific factors, the pumping station designer's best course is to consult the pump manufacturer, who deals with such problems constantly and can temper theory and calculations with practical experience. The same cannot be said of motor manufacturers because they rarely seem to be cognizant of the needs of pumping stations."

 

[MJCronin]

As always, bimr gives clear and accurate advice .....

I believe that this discussion should be expanded into all submersible pumps and into Realistic Sump Sizing !!!

He states:

Submersible motors, cooled by the pumped liquid, can usually withstand very frequent starts—sometimes much more frequently than 20 starts per hour. For such motors, it is generally the starter (not the motor) that limits the frequency of starts.

Because objective decisions concerning allowable frequency of motor starts must be based on such a myriad of site-specific factors, the pumping station designer's best course is to consult the pump manufacturer, who deals with such problems constantly and can temper theory and calculations with practical experience. The same cannot be said of motor manufacturers because they rarely seem to be cognizant of the needs of pumping stations."

My experience with steel mills and power plants has been, watch out for undersized sumps and b*tthole process engineers (BPEs) with extreme sump temperatures !!! Frequently, BPEs will specify a 150F operating temperature for a sump.... This is grossly unrealistic and will make submersible pump selection nearly impossible .... There is no reason for this

Remember, the submersible pump must reject heat to the liquid that it is pumping in the sump!

Submersible pumps get no respect in the pumping world although they are more important than and can cost far more than their ANSI equivalent.

Management attitude? ... "Gotta sump ?... Any size sump is acceptable and Slap a submersible into it and move on !!!"

As a general rule of thumb, I have repeated heard, no more than six starts per hour ....

The sump should be deep and sized for TWICE the maximum expected flow in one hour

My opinion only ...

MJCronin
Sr. Process Engineer