Lifting Station 1

[SDTH]

Hi,

I am involved in the design of a sewage lifting station. Could any one help me with the level setting?
We have considered four levels (as per existing lifting stations) to be sensed by a level transmitter.
The following are the levels - highest to lowest.
1. high level alarm (set below the invert level of lowest inlet pipe to the lifting station)
2. pump start (how much below the high level alarm should I set this?)
3. pump stop (I have used the prevalent formula: t=4V/Q, this seems to be fine)
4. pump trip (minimum submergence. how much below the pump stop level should I set this?)
Is there any international standard or guide to be used as reference?
For item 2, will it be logical to consider a volume equivalent to 5 mins of inflow and select the height accordingly?
Likewise, for item 4, will it be fine to consider a volume equivalent to 5 min outflow (pump flow) and select height accordingly?

One more question..we don't have redundancy for level transmitter. What could be the worst case scenario if the level transmitter malfunctions and pump starts operating below minimum submergence level?.. Probably before the impeller and seal jams due to over heat, will the motor breaker trip? I doubt this bcz the client specifications calls for an over sized motor. Apart from selecting the mtor 10-15 percent more than the end of performance curve power input, the motor is supposed to be derated to 52 Degree celcius.

Note: It is a one duty/one stand by configuration (without duty - assist operation)

Thanks in advance.

 

[ashtree]

I am assuming your level transducer is an analogue 4-20ma type. This should be set up so that if the unit goes faulty the pumps stop pumping. But you should then install a high/high alarm float so that you have some sort of indication that the system has failed.

With regards to the minimum submergence item (4) this has to be set with reference to the pump intakes. This has to be set above the level at which the pump sucks air and stops pumping but below the pump stop level.

The pump start level will be determined by the size of the pumpstation, flow in, pump rate and what you consider is an acceptable number of starts per hour. The configuration of the inlets into the station should also be considered. Generally the inlets are configured to enter well above the bottom of the station and the normal start level would be below that.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[SDTH]

Thank you Ashtree,

The calculations wee done as what you suggested - 10 start/stop per hour, minimum submergence from pump vendor etc.
The grey area is whether we have a specific standard or rule of thumb to follow while we set the levels (volume and there by heights) between pump stop and pump trip, between high level alarm and pump start AND between inlet pipe invert level and high level alarm.

I have attached a sketch outlining the above.

Regards,

 

[ashtree]

With regards to the high level there are a couple of pieces of information you need or you will have to make a guess on.

What is the maximum flow rate into the station and how often this occurs . You will need to consider what happens regularly in this regard and ensure that if the flow in exceeds the pump capacity that you have enough volume in between the pump start and the high and high alarm levels so that you don't get alarms everyday during the peak flow times. Likewise you don't want the standby pump coming in and out all the time either. So you will have to think about the peak inflow and duration. Storm events etc may need to be treated differently.

The bottom pump trip needs to be low enough to avoid nuisance trips but high enough to ensure the pump does not develop vortices and sucks air.

It may seem relatively unscientific but these levels are usually able to be quickly worked out on site if the load into the station is reasonably close to what it will see long term.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[SDTH]

Thanks Ashtree....

We have limitations as it is a retrofit project.. the invert level of incoming pipe and sump base level is fixed as it is an existing lifting station.

Will share a sketch once done..

Regards,

SDTH

 

[LittleInch]

SDTH,

What you describe - setting of stat/stop, alarm and trip points is a complex task as each situation has it's own issues.

Therefore what you tend to find is a general philosophy with guidance - just that this changes between operating companies and it is not good practice to state set % or min distances without looking at the overall system.

The aim should be to maximize the pumping time to reduce pump start/ stops whilst also minimising alarms and trips.

There is a lot of work which goes on now about alarm and trip reviews and looking at the overall operations and the key question for the high level alarm is what can you / do you do if it alarms?? Start the second pump?, turn some inlet off? or do nothing? I don't know so you need to question what the purpose of the alarm is and what time you need before it climbs to your invert level. If there is no purpose and it's just there because it was there last time then set it at min invert level

For the distance from pump start to alarm you need to be clear that 99% of the time the inflow will not exceed the outflow for long enough that the alarm goes off due to rise in level.

If the pump is happy at min submergence, I don't understand why don't set the pump stop at min submergence or maybe 100mm above if you want a bit of margin.

10 start/ stops an hour is fairly brutal and long term this will impact your motor. In general getting this number to as low as possible is a general aim, at least for me).

In the end the ops boys will adjust it to make their life easier so to start with its not a perfect science, just the best you can do based on a set of guiding principles. Just write those down in the operating and control philosophy document so someone else can figure it out later.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SDTH]

Thank you Littleinch,

Well explained. I will work it out and share the final sketch...

Regards,

SDTH

 

[SDTH]

Ok.. I may sound dumb..

We got the discharge line route surveyed...

the pump infact pumps down hill with peaks in between...

liquid level at pump suction centerline 126.83m...highest point level 131.58m... discharge point level..126.3m

Shoudnt I consider static head as 131.58m - 126.83m. The FEED design considers 126.3m - 126.83m and hence came up with a negative static. I don't think this is right, as the pump anyway has to pump till 131.58m. ther is no chack valve or control valve at the discharge point.

The pipeline is like 750m long. HDPE OD 160mm.

Regards,

SDTH

 

[LittleInch]

You need to plot the head of the pump and the frictional losses / arrival head on the same drawing as the profile.

If the head loss line from the pump doesn't hit the high point, then you can ignore the high point in your hydraulic calculation and just look at the end points. It will depend on where on the profile the high point it. Anywhere near the start - shouldn't be an issue.

Normally I would say elevation differences like this are irrelevant, but I know these types of systems work on small head losses.

If you past the profile nand also advise what is the flow rate, and head of the pump it will be easy to see if there is an issue there or not.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SDTH]

Thanks Littile inch..
We will workout the hydraulics.
Meanwhile, I'm listing down few points. Please check if my understanding is correct...

We kept 20 cm from high high alarm level to the lowest inlet pipe invert level– to allow some time for the alarm to sound and operator to come and take action.

What will the operator do when he comes?..nothing much..he cannot go and shut down all toilets…but in few cases he can do his part as I am writing below..
In my understanding, the alarm can sound in few cases…
1. both the pumps malfunction and level goes up (because if one pump malfunctions, the control panel starts the other automatically)
2. the level transmitter skips to sense the pump start level and pump doesn’t start..the level goes up to reach alarm level. Transmitter picks up this level, signals the control panel and alarm sounds..
3. The control panel malfunctions..it doesn’t start the pump automatically.
4. The inflow is more than pumping capacity..
5. The pipeline is clogged somewhere and pump cannot pump.
For case 1, the operator may have to call a sewage truck and keep it on standby.
For case 2 and case 3, the operator can manually switch on the pump from the panel - in hand mode.
For case 4, we are checking the adequacy of electrical side to start both the pumps together to have more flow. Case 4 is not gnna happen. The flows we took to size the pump was during shut down time when all the staff are in their accommodation and potable water use is at its maximum. On top we applied peaking factor of 3.
For case 5, it will be a rare case and will require shut down.
Please let me know of any other scenarios.

Regards,


SDTH

 

[LittleInch]

looks pretty comprehensive to me. Item 5 could also be pump clogged and flow less than it should be, but same consequence.

If your item 4 is as you say then pump start can be quite close to the alarm setting - say 10-20 cm as there seems little chance of inlet flow being greater than pump flowrate, but that's a judgement call depending on what the volume is per cm and could be adjusted down if the alarm keeps going off.

Only having one transmitter / level alarm is putting everything into one point of failure - better off to duplicate this somehow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SDTH]

Thanks little inch..
Redundancy for transmitter is a weak point..
But somehow the engineers who did the FEED design did not look into it.
We are onboard as Detail Engineering consultant assigned by the Contractor who bid the project keeping one transmitter in mind. One transmitter costs USD 6000. I took up this point but Contractor in noway agrees to it and Client wont pay a penny extra.
Somehow i convinced Contractor to provide an additional float switch for redundancy of pump trip level setting - atleast the equipment is safe.
Thanks for your suggestions..
Regards,

SDTH

 

[LittleInch]

If your tx is radar type you might not be so bad, but any float system or anything which can get fouled will create a bigger issue for you.

Anyway glad you've got the info you were looking for.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SDTH]

Hi little inch...

Things have taken a new turn..
Client checked the proposal for providing an additional float switch(for pump dry run protection redundancy) along with the level transmitter; and guess what, Client omitted the level transmitter an has asked to use four independent float switches instead (one for each level)..
Tell me, the float switches supplied by pump vendor like KSB, Flygt - are they of industrial grade or we should insist on instrument vendors like Endress and Hausser?

Regards,


SDTH

 

[ashtree]

SDTH

Most of the float switches you have mentioned will give satisfactory service. The question will be how they are installed and how well they are looked after.

Whatever way you install them they need to be prevented from floating around freely in the pump station. Some slide conduit down over the cable leaving only a short bit plus the float exposed. Some fasten all the floats to a weighted spreader bar that is located just above the highest float. Some put weights on individual floats. Some tie them to something at the bottom of the station. I don't recommend the las system but i have seen the others work okay but whatever way you choose to install them they must be prevented from floating around freely and becoming tangled. Ideally the floats can be pulled up from ground level without having to enter the wet well.

Many floats are damaged from being hosed from ground level and smashing against something solid. Having them fastened as above reduces this.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[LittleInch]

I agree with ashtree. Should be good enough for the duty, but the devil is in the detail of the float type, it's support structure and how and where it will be mounted.

Inside a pit with a lot of swirl and water movement floats can be come jammed in one position or another and fouled.

You may want to consider a differential pressure transmitter rigged as a level transmitter / switch.

floats might work better inside a stilling tube and you can remove for cleaning / maintenance.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.