No flow from a grit pump

[oldfitter]

Guys,
We have a submersible centrifugal pump sat in a grit king hopper. It pumps up a vertical 150mm pipe 6m long ( or it should )before gravitating into the grit classifier. Ops reported that it will not pump. They've assured me that the hopper is clean, and the discharge pipe is clear. We removed the pump and it is clear. The impeller is 181mm diameter as it should be. The rpm is supposedly 940. The closed valve head is 4.8m and the static it has to overcome is 2m. The flow ought to be 15 lps. If there is any grit in the bottom of the hopper it should be loose as a blower agitates in every 5 minutes for 5 minutes. When the pump was being removed, we left it part submerged and switched it on. There was barely a ripple in what is cleanish water near the surface. My question is, can an electrical fault cause the pump to run slower than expected and by enough to reduce its closed valve head. Its hard to tell without a tacho but it seems visually slower than i would have expected. It ran very quietly and smoothly with hardly a starting kick . I,ve asked ops to fully drain the hopper in the meantime.
The pump is direct on line i think. Definitely not a VSD.

Thanks in advance.

 

[bimr]

If the wiring is incorrectly connected, the pump may be running in reverse. Was the pump working previously?

There may be a gap that is too wide between impeller and bottom plate from pump wear or the pump may be packed with sand.

The pump is throw-away. Go ahead and replace it. Consider the installation of a pump that will require less maintenance than a submersible. Self priming centrifugals are used for this application.

http://www.grpumps.com/product/pump/177

 

[oldfitter]

Thanks for your reply bimr. I checked the direction and it's correct. The pump is a vortex pump with a recessed impeller which looks to be in good condition. Just thought there may be some obscure electrical fault causing it to run slower than normal. Hopefully we can get the hopper drained sometime soon to rule out the possibility of something stuck in the bottom. Will repost once I have the answer.

 

[1gibson]

Head will be reduced by the square of the speed ratio. 1/2 speed, 1/4 of the head.

2m/4.8m = 0.41667
0.416667^0.5 = .645
.645x940 = 606.7 rpm or slower to not be able to overcome the static head.

Something rubbing (pump problem) should not slow your motor down that much. Worn wear ring clearances in the pump might have you losing some efficiency and account for some of the lost head, but even if you call it 700 rpm, no mechanical problem is going to reduce the speed that much without frying the motor in a very short time.

Impeller isn't collet mount, is it?

 

[itdepends]

Snapped/broken shaft or coupling between the motor and impellor?

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

 

[Artisi]

"When the pump was being removed, we left it part submerged and switched it on." --- Was there any flow from the pump?


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[oldfitter]

Thanks for all the replies so far. To summarise.

"Impeller isn't collet mount, is it?"............ No it is keyed on and has a retaining bolt. I,ve not removed the impeller yet but assume the key is in place.
"Snapped/broken shaft or coupling between the motor and impeller"............ as above.
"When the pump was being removed, we left it part submerged and switched it on." --- Was there any flow from the pump?"......... Yes but not as much as i would have expected in that, the pump is drawn up a centre tube approx 1m diameter. The flow from the pump outlet barely reached the tube wall. I didnt observe this but the operators said water was coming out from the discharge but they stood back expecting a lot more force and were surprised by the lack of it.

 

[Artisi]

Wrong rotation - re-check, not the first time a pump was running backwards.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[keyepitts]

Lost a leg from a three-phase motor? Slipping coupling on motor or pump shaft (sheared key)?

 

[oldfitter]

Guys,

Emptied the grit trap yesterday to find the bottom of the hopper to be completely full of compacted grit plus other and pieces, a drain rod amongst other things. While the pump was out we checked the impeller, the key was in place and secure. The speed was checked with a tacho and found to be correct. Looks like the pump has been sat on the grit for months. Looking forward to a skip full of grit.
The control philosophy built into the program utilises a night time mode where grit removal is automatically set to 3 hour intervals. This is a possible cause. Going to set it the same as during the day and monitor.

Thanks again for all your replies.

 

[Artisi]

So how do you explain "When the pump was being removed, we left it part submerged and switched it on. There was barely a ripple in what is cleanish water near the surface."?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[oldfitter]

Cant explain that Artisi, other than i didnt see it with my own eyes. Ops did the initial cleaning of the hopper and said it had been fully cleaned when it hadnt. They also told me about the poor flow when it had been lifted and run while only just submerged. I dont know the level of submersion and wonder whether it could have been drawing air in or simply that the recessed impeller was not even in the water. We should be in a position early next week to run the pump sat on its stool with a tank full of cleanish water. i will then put an ultrasonic flow meter on the riser pipe to establish the flow rate. I think it should be in the region of 15 Lps. will report back.

Thanks

 

[Artisi]

Sounds pretty normal, believe none of what you are told and half of what you see.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[psv1990]

Check impeller wear again. With a votex recessed impeller, the wear will not show up in impeller diameter but as wear on the blades.

 

[oldfitter]

Guys, final update. Once the hopper was cleaned out thoroughly and the pump replaced, it pumped perfectly well removing grit. However the root cause of the grit removal system was found to be a faulty simocode unit in the control panel. The open / closed signals from an automated wash water valve on the classifier screw were not being recognised by the PLC. The classifier thought the valve was neither open nor closed and as such was stopped which in turn stopped the pump. Ops did nothing about it and the grit hopper filled up and the grit compacted. I think that during their investigation, they lifted the pump and replaced it a few times and dislodged grit prevented the pump seating properly. Then they gave up and bypassed the grit system altogether.
The wash valve failure should not have caused the system to fail. This has now been written out of the program. Now if it fails, an low priority alarm is raised but the grit removal system continues.

Thanks for all your posts.

 

[bimr]

Thank you for posting the update.