Pulsation Dampner Charge Pressure

Both questions are best addressed by the design and operating info for the damper. Please refer to that documentation. Having said that the charge pressure has to be maintained above line pressure for the damper to work. If the bladder material is compatible with air it should be ok. However nitrogen is preferred as it is clean and dry. Most air sources are not and water with oxygen will induce bladder rot.

Sorry but nitrogen is used because it is inert. Mixing oxygen and oil under high pressure conditions can induce explosions. Yes the bladder seperates the oil and gas but not all the time!

Excellent point, if the pump is pumping any kind of hydrocarbon, then N2 is the preferred gas for the bladder.

Good answer vote for you.

Hi Steve S

Thanks for that cobber!

I have a query for you regarding your previous answer above! If the accumulator pressure is maintained above line pressure, the accumulator bottom sealing button will remain closed and will, in effect, do nothing! It's only chance of helping a system in this case would be from e.g. external effects such as sudden loads exerted on a cylinder.

As the OP suggested that the accumulator was to damp (and I'm assuming here) pulsations from a PD Pump (positive displacement, probably piston type pump???) then, the rule of thumb would be to run the pre-charge at about 80% of line pressure thus keeping pulsations flowing into (and out of) the accumulator giving a smoothing action to the system.

Any thoughts?

Apologies, I oversimplified the issue by a long shot. What I really was trying to get at was that damper pressure has to be high enough. Often the maintenance of these things are such that the N2 pressure is never right and they don't work. For more info on set pressures see the following:

http://www.qhp.co.uk/pdfs/HydraulicDampers.pdf

The whole point of the bladder is to dampen pulsations. A pd pump pulses flow into the line and due to acoustic resonance the pressure pulse can be amplified to the point that the peak pressure is as much as twice line pressure, and the valley of the pulse can go right down to the vapor pressure of the liquid being pumped resulting in cavitation. You have to have sufficient charge pressure on the bladder, or the bladder won't react dynamically.

The damper as expected adds acoustic damping to the system. In essence the gas inside the bladder makes the damper vessel look (acoustically) like it is hundreds of times larger than it is. This saves having to put a huge vessel on the end of the pump to prevent the pressure pulse being a problem. However if the bladder is collapsed because there is not sufficient pressure in it, then the vessel looks acoustically like the small vessel it is and it does nothing.

Actually the best way to charge the bladder is to measure the pulsation while charging the bladder and optimise the charge pressure based on pulsation levels. That is rarely done.

Yes, Steve S, I agree! However, if the frequency of the pulsations (as per a 9 piston pump @ 1440 rpm) are that high then a bladder type accumulator is not the answer and the OP should be considering a diaphragm type accumulator that has a far faster reaction time!

I hope you follow my ramblings!

I don't think the OP knows the depth of the muddy puddle he/she has just stepped in!

That seems certain...

Pressure u require is depends on the volume of blader. But normally it will be more than 4bar.

Even it moves very high in some cases.

Your Idea is perfect but u need to work very hardly, because u need to maintain the pressure. See partial pressure of nitrogen is very low and hence once it expose to atm pressure then it wil suddenly vapourised and hence the physical pressure increase. that is why generally it is used in ur application. See air is combination of oxygen, nitrogen and other gases. Here in ur application they never prefer the compressor as nitrogen make very high pressure and suddenly it forms in gases. So they will get the exact pressure as per their volume of blader. If u use air the same thing wil happen but u need compressor. U r changing their system. If u want it u can change.

See air partial pressure is not low as of nitrogen because.

P _air = P _nitrogen + P_oxy+ P_{other gases.}

P is partial pressure.

See manufacturing wise u will follow the same thing. If u use nitrogen then for getting nitrogen u need to use electricity for liquefying. And in ur case to maintain the pressure of air u need to use the compressor type systems wherever u needed. So ultimately the same. But to maintain nitrogen carries more efficient rather than air due to loss in extra mechanical systems like compressors, bendings, length of pipes, compressor efficiency

.

Some theory related with partial pressure.

In a mixture of ideal gas each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture.

In chemistry, the partial pressure of a gas in a mixture of gases is defined as above. The partial pressure of a gas dissolved in a liquid is the partial pressure of that gas which would be generated in a gas phase in equilibrium with the liquid at the same temperature. The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases will always flow from a region of higher partial pressure to one of lower pressure; the larger this difference, the faster the flow. Gases dissolve, diffuse, and react according to their partial pressures, and not necessarily according to their concentrations in a gas mixture.

The partial pressure of an ideal gas in a mixture is equal to the pressure it would exert if it occupied the same volume alone at the same temperature. This is because ideal gas molecules are so far apart that they don't interfere with each other at all. Actual real-world gases come very close to this ideal.

A consequence of this is that the total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of the individual gases in the mixture as stated by Dalton's law.

No offense intended, but I don't think you understood the application at all. The nitrogen is in a bladder, it is not being injected to the pumping fluid. See the link I posted above.

I strongly suggest you visit this site((( http://www.newequipment.com/303/ProductDetail/59760/303-8500/Free_Software_from_Simplifies_Sizing_and_Selection_of_Hydraulic_Accumulators.aspx)), which is Parker Hannifin. This application guides you through the process of properly sizing and charging the accumulator for your particular system

Pulsation Dampers are found in different applications. Although dampers are used primarily in hydraulic circuits where, as mentioned by many before me, nitrogen is used instead of air to avoid any oxidation of metal components and the rubber. Pulsation dampers are also used in Closed Cooling Water Systems where the water is contained inside the bladder and Nitrogen is again used to dampen by filling the space between the bladder and the vessel.Here too air is not used to prevent corrosion of the vessel internals. Another use of Pulsation Dampers is also in chemical dosing applications where positive displacement pumps are widely used(typically in Power Stations) where air or nitrogen can be used since typically volumes are low and stainless steel vessels are used. As someone earlier remarked, the thumb rule is to precharge to a pressure of approx. 80 to 85% of the operating pressure for effective dampening. Again, most manufacturers prescribe the pre-charge pressure on the vessel itself which should be followed. It is also relevant to mention that pre-charge of nitrogen or air should always be done with the pressurized medium completely de-pressurized(i.e. the working medium drained completely). In addition, accumulators or dampers come in various constructions which include bladder type, piston type etc.

Hope the above explains the initial query posted. The above comes from my practical experience of over 20 years in Power Plants.

Ashok

Hi Guest

The reason air or, even worse, oxygen is not never used while damping a hydraulic system is NOTHING to do with oxidation. It's ALL to do with the potential to create a bomb! As the gas is contained (generally) within a bladder, oxidation is not of prime importance.

An accumulator that explodes is as bad as any fragmentation bomb in the NATO arsenal! This is not a subject to mess about with by inferring that something other than an inert gas will do!

I think you may be getting confused with other applications found in power plants that I agree, exist!

I have never heard of explosion being a possible risk in accumulators unless you are dealing with explosive media!! I do not see the relevance of the earlier comment in say a system involving closed cooling water or a hydraulic system of moderate pressure range!! I definitely know for sure that air is also used in pulsation dampers in the discharge of positive displacement pumps in acid dosing system and that is from personal experience. I think Asif's initial query was related to a similar application in a system involving positive displacement pumps!!

Ashok

I have never heard of explosion being a possible risk in accumulators unless you are dealing with explosive media!

Then, I trust that the pictures below will convince you. The important thing is that you learn and definitely NEVER advocate the use of anything other than INERT gas when it can mix with high pressure oil systems. Similarly for the inverse, never oil the threads of gauges that come into contact with high pressure gasses such as oxygen!

This is accumulator bank from below!

And this is the view of the same accumulator having penetrated a 500mm factory concrete floor.

I definitely know for sure that air is also used in pulsation dampers in the discharge of positive displacement pumps in acid dosing system and that is from personal experience.

Yes, ACID not OIL!

I think Asif's initial query was related to a similar application in a system involving positive displacement pumps!!

Yes, I think you are correct BUT WE CAN NOT RECOMMEND SOMETHING that may be UNSAFE AND DEADLY in this forum! If Asif's system was oil...............!

I really do hope this post clears up any confusion for you!

Gents,

Thanks for participation and contibuting your knowledge and experience. I would like to close this discussion with the comments that it is always better to follow the instructions of OEM as the guidelines given are based on years of experience and experties regarding the products they manufacture and market. In nut shell Dampners must be charged with inert gas that is Nitrogen whether on oil or acid service.

My second question is not answered specifically so far that is how much pressure is required to maintain in bladders. According to my knowledge it should be around 115 to 120 % of line pressure (operating discharge pressure) of pumps. Plz correct me if I am wrong.

It should be about 80% - 85% of line pressure. Read #4 from Beej50 to obtain explanation!

You could also go to the Parker web site listed in a comment above to get a different perspective!

Precharge pressure is somewhat subjective. At 100% of system pressure any pulses above that pressure "maybe" absorbed by the accumulator.

If the precharge is 80-90% then some system pressure is absorbed by the accumulator (20 or 10% of accumulator capacity) such that if the system decreases pressure the accumulator can "push" additional flow to make up the loss.

It all depends on what you wish to accomplish..... absorb spikes or decrease pulses or others (ie: maintain system pressure). Some of this is also involved in the placement of the accumulator in the line. Closer to the pump, at the accuator valve, 90* to the flow or inline to the flow.

Different types of accumulators are suited better to different types of jobs. Bladder, piston, diaphram or even weighted type of accumulators can be better suited to some jobs.