Offshore water pump. 6

[jamesss]

The project;

I am currently working on a pumping system to draw/pump water from the sea, to the access level of offshore wind turbines.The sea water will be used to wash off bird waste that accumulates in large quantities causing problems for offshore operations.
The pump will be a self priming centrifugal pump 230 v 160L/min 5 Bar.
The suction line is 7 meters maximum length, 1.5 inches diameter pipe.The discharge line is also 7 meters but has a diameter of 1 inch.The pipe diameters are matched to the inlets and outlets of the pump design.
There is a non return valve on the suction side 3 meters from the pump, this is to maintain water in the pump body and discharge line to assist with priming.
There will be a control circuit activating the pump at high tides only and during night time i.e. one high tide per day.

Testing of the self priming pump, suction hose,non return inline valve offshore.

During high tide the pump was situated on the resting platform (7 meters from sea level to pump inlet).the pump body was then filled with water ready for self priming.The suction hose was placed into the sea water and the discharge hose was run uptothe turbine access level 7 meters up.

When the pump was activated, i could see that the pump was extracting air from the suction hose through the pump housing and into the discharge hose, the air bubbles and water mixture would rise so far and then fall back into the pump housing. After 5 minutes i turned the pump off and could see the trapped air in the pump housing beginning to naturally rise up through the discharge pipe and out into the open.When reactivating the pump after all the air had vacated the pump casing, the pump performed as it should,displacing water from sea level upto the turbine access level at a rate of 80 liters per minute.

Each time i turned the pump off and then back on, it would activate and start pumping straight away however, when i turned the pump off and introduced air into the suction pipe to simulate the tide going in and out, the pump would not prime and would continue to collect a water/air mixture in the pump casing and fail to prime fully, leading to failure to pump.As mentioned, if i turned the pump off then on to allow the air to escape the casing then the pump would perform faultlessly.

After investigating further into the reason the pump was failing to prime first time, i came to the conclusion that the head of water on the discharge line was not allowing the air/water mixture to escape resulting in air binding thus loosing suction.

My questions,

Is this something anyone has seen before?
Is my conclusion plausable?
if so, would an automatic air breather installed on the pump casing be of use for my problem?

I don't really want to re-configure the control circuit so the pump activates twice as i don't see that as a sound solution to the problem at hand.

Any help will be much appreciated!

 

[LittleInch]

Jamesss,

You maybe caught me on a bad day, but sometimes we have different ideas based on experience and design which can challenge certain assumptions. I've looked at submerged and above water pumps for seawater before and the self priming systems have issues IMHO which are negated by submersible pumps.

For this type of installation I would have thought you're looking at attaching a caisson / riser to the tower base and then you hang the submersible pump off the top and withdraw it in the same manner to service it / remove dead fish/plastic bags etc.

I agree marine growth, shell fish, fish and plastic, etc is a major issue when you deal with seawater. This applies whether you dip a tube in and out of the water ( is this tube there all the time but sometimes out of the water?) or have a submersible unit. Your system needs to allow for this and hopefully not clog up on a regular basis.

Pump head, power etc are identical for the same system.

However,if you're happy with a self priming system and need to get it to start reliably, try one of the suggestions above - either divert flow at low head to establish flow then turn it off on timed control or try an air eliminator for which you may need to install a double U bend in order to get air to collect to vent off.

But let us know what you decide and in time if it works.

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

 

[EdStainless]

There is an entire industry dedicated to seawater intakes, pumps, and anti-fouling treatments.
Putting a sub- pump in a casing and making sure that it is well below the tidal level will go a long way in assuring longer life and more consistent operation.
In fact the deeper you go the easier this is, because the water gets colder and the oxygen levels go down, both of which slow the marine growth.
Have you calculated the lateral force that waves will apply to your 1.5" intake pipe? I don't think that there is a material that will stand up to this.
But inside of a piece of 6" or 8" casing you should have a much better chance. I know, more material, heavier, costs more. But you would like for this to work and maintenance would cost 10x the original price.
If steel is used for the casing it can be costed with anti-fouling paints. Of the whole thing can be made from Nickle Aluminum Bronze or Cu-Ni. Both of these alloys will resist corrosion and are high enough in Cu to be fouling resistant.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[jamesss]

Ed stainless, thanks for your reply.

If someone recommended a sump pump that will have a longer life than 5 years in saltwater and can be installed in such a way whereas marine growth, mussels/barnacles don’t pose a threat that has a head of 50 meters/180 l/m then I would be interested.I have just came off the phone to DAB who are an international pump wholesaler and they have said I will struggle to find such a pump.
Any pipes exposed to the sea conditions are going to be enclosed in a robust sewage like piping used as a form of conduit.This will enable any pipes in future that are defective can be extracted from the conduit without having to work at sea level.

 

[TugboatEng]

Marine growth on the outside of the pump has no affect on performance or longevity. You're going to have even bigger problems with marine growth in your suction piping than you would with a submersible pump as the sub pump is pushing through the barnacles instead of sucking through them.

 

[jamesss]

Hi tugboat, I don’t see how that makes any difference, water is being sucked from the sea in both systems..the same amount of liquid will be displaced up the suction pipe (on self priming pump) as up the discharge pipe on a sump pump?

Just to be clear, I’m not opposed to using a submersible pump at all, I just can’t find one that would fit my needs?
If the suction pipe is treated with anti-fowl paint and is easily accessible via the mentioned conduit system then i don’t see a problem.There will obviously be a fine strainer on the suction inlet pipe.
If a manufacturer on here can supply me with a sump pump that will meet all my needs then there is the potential for a nice pay day.
I am told over and over salt water will destroy anything you submerge within about 3/4 years.
Can you blame me for not going down the submerged pump route?

 

[miningman]

Well , changing out a submersible every 3 or 4 years might seem very much like the lesser of two evils if you are still struggling to maintain prime on the existing system in say 6 years time. I wonder what a cost comparison might look like

 

[LittleInch]

jamess,

Just to be clear here, what is your actual duty point?

Your stated 50m / 180l/m are max amounts, but not at the same time.

So for your chosen pump 50m differential head is at a flow rate of less than 20l/min whereas 160 l/min is at a head of 20m.

So which one is more important?

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

 

[jamesss]

Hi little inch,
I don’t really understand what you are asking.I require around 3 bar of pressure at a rate of 140l/m at the access level where the bird waste problem is.The water supply will then be connected to various spray nozzles which will disperse the bird waste.
The 50m head 180 l/m self priming pump I already have will accomplish this.

 

[EdStainless]

I hate to say it but 5 years in raw intermittent raw seawater service is nearly unreasonable.
This pump would last longer running all of the time.
I think that you stand a better chance keeping a pump intake clean than a suction pipe.

I did see an installation once that was on a pier. Every time that the pump shut off a small valve cycled and allowed it to suck some biocide back into the line and out the intake. I seem to recall this system using 10-20ml per cycle. In their case 1gal lasted a year.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[danschwind]

I agree with all the colleagues suggesting a submersible pump - it is the right option in this scenario.

I once did a project where I installed a pretty large submersible pump in a FPSO. The caisson where the pump was installed received the injection of an anti-fouling substance that would impede, or at least diminish, the formation of marine growth. It may be an option if this is the main issue of using a submersible pump in this case.

Daniel
Rio de Janeiro - Brazil

 

[LittleInch]

The pump you have produces 25m @ 140 l/min. If your access level is above the pump level then you're either not getting 3 bar or a lot less than 140 l/min.

To be able to source a pump you need to specify a duty point, not just the extreme end points of e pump curve.

Your chosen pump will not produce 50m @ 180 l/min.

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

 

[jamesss]

Thanks for the info guys it is very much appreciated.


My mistake the pump is rated at 160l/m at 50m

By the time you take into account around 7 metres suction head and 7m discharge head, pipe resistance and non return valve resistance I am receiving around 130-140l/min just under 3 bar
Which is more than adequate.
I would require a sump pump to to match this.

If anyone wants to point me in the right direction I would be much appreciated.

The application of anti foul could be easily applied periodically.

 

[LittleInch]

For the last time, your pump is NOT rated at 160l/min at 50m differential head. You are misunderstanding how pumps are rated.

If indeed you are getting 130 l/min, the pressure/head at your discharge point according to the JBR 4M pump curve ( as you're operating at single phase 230V) once you take off the static head is giving you about 12m at the end point. Allow a little bit for frictional losses and the pressure at your +7m elevation is about 1 bar. This seems to be acceptable.

Therefore the duty point to look for in submersible pumps will be approx 130 l/min at 25 to 30m head.

It does look like you fall between a sump pump which tends to be about 10 to 15m and high head down hole borehole type pumps which are >100m, but there will be some units which can do this.

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

 

[jamesss]

Hi
Litleinch.

Thanks Again for your reply and information shared.

I appreciate your break down and correcting me on the pump rating versus duty point values as this is obviously critical.Whilst i am certain i am getting 130 l/min (which i was able to measure) the pressure must only be at 1 bar.
I will add, it is an unknown at what pressure i will require at the outlet spray nozzles however, this is part of the point in constructing a test rig on one turbine for a number of months before rolling it out to the fleet.I suspect 1 bar will be adequate as a good storm goes some way to cleaning the access areas as it is.

I will take on-board the advice given on here and will source a salt water submersible pump for test.
I am still dubious on the life span of these pumps given the conditions,as this was my ultimate reasoning for designing a system with minimal exposure to such a harsh environment using a self-priming pump.

I will keep you posted on my progress and post some videos/pictures.

 

[LittleInch]

Quick question. On your test, what did you have on the end of the pipe to control flow?

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

 

[jamesss]

to control or to
measure the flow?

 

[LittleInch]

Control

If you just had an open end pouring into a bucket or something then you didn't have a pressure at the end of your pipe. As soon as you introduce a restriction to get a jet of water to clean of the bird poo you start to go left on the pump curve and hence may not get your 130l/min.

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

 

[jamesss]

Hi Littleinch,
yes just an open container with a timer to get the l/m is all i had to hand.
I see what you are saying about introducing a restriction however, if my duty point is 1 bar at 130 l/m and i match the flow rate of all the nozzles combined to 130 l/m ensuring the nozzles are also rated at 1 bar then should i not expect to maintain the duty point pressure/flow overall?

 

[LittleInch]

OK, What I think you did and please correct me if I'm wrong, is you have a pump with about 7m lift (suction) to your pump, another 7m up to the end of the discharge hose which was open to atmosphere and into your vessel.

Hence the pressure at the end of your discharge hose is essentially 0 barg.

So you have 14m of lift plus what seems like about 7 - 10 m fictional losses ( you're running about 4-5m/sec in your discharge hose)

This equates quite well with your 130l/min on the pump curve.

The issue now is that if your spray nozzles impose an additional pressure loss / need say 1 bar to operate properly, this adds 10m to your duty. On your pump curve you now only get 80 l/min. In reality there's a bit of give and take ( frictional losses reduce so you might get up to 100 l/min.

The devil is in the detail of the pump curve. the risk is that too many nozzles will result in low end pressure and low veljet velocity. So you end up more with a sprinkler system than a jet wash.

What you really need to do is test one of your nozzles / spray heads and see how much pressure/head it needs just upstream to develop the spray velocity you are looking for and measure the flowrate from each nozzle. Then figure out how many nozzles. This gives total flow needed.

Then add the static height from the sea level to your nozzles
Then work out frictional losses in your pipes and fittings.

Add it all up and that becomes your pump duty at the total flowrate from the number of nozzles.

But things never work out exactly, so allow for turning off or adding nozzles to your system when you test it in earnest to get the spray velocity you need to remove the guano.

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

 

[jamesss]

Hi Littleinch,
Would it be possible to email you some further details and clarify some of the points we have discussed so far?

jamesss48@hotmail.com