Accumulator suitability 1

[CarlosEstinos]

Hello,

I've recently taken over a project at work from someone who has left our company. The project is to design and install Hydraulic cylinders onto large gate valves.These cylinders will act as actuation for the valves.

I've attached a the current circuit diagram.

If the system was to suffer from a power failure then the accumulator must fully actuate the cylinder in 35 seconds.

I'm scrutinising the schematics as I want to be sure this accumulator is fit for purpose as it will be an expensive mistake if it isn't!

Could anyone help or point me in the right direction as to what sort of calculations I need to make in order to clarify my concerns?

Any feedback on this matter would be gracefully received.

Thanks in advance.

 

[hydromech]

What's the accumulator pre charge pressure?

What's the max load of the cylinder?

You will need these to determine how much oil you can get into your accumulator.

 

[CarlosEstinos]

Hello, thanks for replying.

the pre charge pressure will be approximately 65Bar.

The Max load will be 125KN

 

[hydromech]

Ok, so your effective oil volume is about 30 litres.

Work out what pressure you need to generate that force with the cylinder you have.

Assuming isothermal compression the simple calculation is P1 x V1 = P2 x V2.

P1 is precharge
P2 is max system pressure
V1 is 30 litres
Rearrange to find V2

V2 is the volume available at pressure.

Cylinder volume is 16 litres approx.

This is all in my head and typed on iPhone in Schiphol airport. I'll confirm more later.

Adrian

 

[CarlosEstinos]

I shall be visiting schipol myself in a few weeks when I go to visit my sister!

Again thank you for spending the time to reply.

I perhaps need to clarify a couple of my figures because I'm not entirely sure I'm correct. Fmax is potentially 125Kn which occurs at the end of the stoke. The rest of the stroke will require alot less force, as there will be alot less resistance.This figure would probably be around the 50KN mark for 95% of the stroke duration. I've calculated that the pressure required for Fmax is roughly 70Bar , and from that I've derived a pre charge pressure for the accumulator. My assumption is that it should be 90 % of the pressure required for Fmax (this is based on information I've been given by a hydraulics fitter) its not something I'm treating as gospel! So that's how I've come to the figure of 65 bar for the pre charge pressure?

You say 30 litres is the effective volume, what exactly do you mean by this? And how is that figure derived?

I've calculated V2 based on my assumptions and that gives me a figure of 28litres. What is that telling me?

Sorry for the barrage of questions! Hope you have a safe journey!

 

[hydromech]

I assume it's a 35 litre bladder accumulator, in which case the bladder and the anti extrusion take up some of the space. Its safe to assume 30 litres of usable oil volume.

Ultimately you need to know that when the accumulator is fully discharged, there is enough pressure to drive the cylinder...wherever it is in its stroke.

I see there is a counterbalance valve on the cylinder. There is also the pilot pressure ratio on that to consider. Do you know what it is set to and do you know the pilot pressure ratio?

The pressure regulator is set to 140 BAR, that is the limit of the system pressure. What is the pressure switch set to, does that switch the vented relief valve off...what pressure does it go off at?

I'll work the acc details out based on your details and get back to you.

Lots to think about...I like this!!!

Adrian

 

[CarlosEstinos]

Yes you are correct to assume that it is a 35 litre bladder accumulator. The ratio on the counter balance valve is 4:1.

The pressure switch, is to maintain the oil pressure at the accumulator, so when it falls below 140bar the motor/pump kicks in to re pressurise the oil side of the accumulator up to 140Bar.

None of the figures are set in stone regarding this system, as we are yet to test it, but the accumulator as you say must be able to drive the cylinder wherever it is in its stroke.

I look forward to your comments.

Carl

 

[hydromech]

The pre charge needs to be 90% of the minimum pressure required to the drive the valve. That way you will ensure you have enough gas driving force to move the actuator.

Loading your acc to 140 BAR will mean you have 274KN of force available to get the valve moving. I would imagine that's enough.

You need to know what the counterbalance valve is set to, with a ratio of 4:1, the minimum pressure will be that valve divided by 4.

Let's say your 65 BAR is 90% of the minimum pressure. The minimum pressure would be 71.5 BAR. That would give 126KN.

Your acc is cycling between 71.5 BAR and 140 BAR.

P1 = 65 BAR
P2 = 71.5 BAR
P3 = 140 BAR

V1 = ?
V2 = ?
V3 = ?

Assuming isothermal compression to get to P3.

P3 V3 = P1 V1 n = 1

Therefore

V3 = P1/P3 x V1
V3 = 65/140 x V1
V3 = 0.464 V1

Working on the safe side, lets say the accumulator exhausts adiabatically. That is the nitrogen cools and shrinks as it decompresses. (n = 1.4)

Thus P3 V3^1.4 = P2 V2^1.4

V2 = (P3/P2)^1/1.4 x V3

V2 = (140/71.5)^1/1.4 x 0.464 V1

V2 = 1.616 x 0.464 V1

V2 = 0.749 V1

The volume of liquid stored = the volume of the gas that pushes it out.

Your cylinder need 17 litres of oil to fully extend.

17 = V2 - V3

17 = (0.749 V1) - (0.464 V1)

17 = 0.285 V1

V1 = 17 / 0.285

V1 = 59.649 Litres

This suggest that your 35 Litre accumulator is not big enough with these pressures. These pressure need to be verified, but you can use the same steps to verify. Its always best to assume adiabatic expansion.

Let me know how it goes.

Regards

Adrian

 

[hydromech]

A further update...

I have uploaded a simple spreadsheet that can be used to see what options there are.

You can adjust P2 and P3,also the volume of the cylinder.

Looks like the best way forward is to reduce the cylinder volume which make sense as you have plenty of options working at 140 BAR. You can drop the diameter and that will reduce the volume required.

If you don' want to reduce the cylinder bore diameter, you need to use a 54 litre accumulator.

Regards

Adrian

 

[CarlosEstinos]

Thanks for that, it certainly seems as though ill be needing a 54 litre accumulator based on your calculations.
I had come to that conclusion based on information id received from an accumulator supplier, but I couldn't be sure as my thermodynamics is a bit rusty!
Ill paint the picture a bit further.
I've got two systems, both are virtually identical, the system I’ve attached is to actuate a 900mm gate valve, the other is a 700mm. The only differences are the motor size and pump size and obviously cylinder sizes. Both circuits have been spec’ed with 35 litre accumulators. The smallest cylinder has a 10 litre volume (full bore) so based on your calculations it would appear that a 35 litre accumulator is fine, that’s based on P1=45 P2=50 P3=140.

The problem I’ve got is these cylinders are being mounted to pedestals which in turn are mounted to the valves. These pedestals are being manufactured as we speak (not literally,unless they are working on a Saturday!) and have specified mating flanges for the hydraulic cylinders. At this stage I can’t change the cylinder size.
The stroke length and the force it can produce are the most important things for the cylinders. I would have loved to have investigated using different smaller bore cylinders as a means of reducing the accumulator size.
I’ve been tasked with making this project work so unfortunately for me I can’t tinker too much as the specification is at fairly advanced stage. But being the way I am I like to make sure everything is correct and the accumulator size was certainly something I needed to clarify, as I thought the force calculations were wrong and made using not so conservative assumptions. Buying the incorrect ones and finding out they were wrong when testing wouldn’t have been good!!
I’d like to ask a few more questions.
What thermodynamic laws are you using? Regarding the calculations you’ve made, do they account for temperature change?
Also how do think I’m able to factor Time into this. As the minimum required time for the cylinder to actuate on the accumulator is 35 seconds.
Thanks for your input on this matter, it is invaluable.

Carl

 

[hydromech]

Hi Carl...

The laws applied are just the gas laws. The adjustment is made with the isentropic exponent.

With isothermal compression the gas temperature remains constant so the value of n is 1. For decompression, which is generally faster, the gas cools and contracts and compensation has to be made to make sure that when the gas is cold there is still enough energy in it to push the oil out.

Where n=1 for isothermal compression, n=1.4 for adiabatic expansion.

As far as the cylinder velocity goes, that is dictated by the pipework and the valves in the system. It is usual to install a pressure compensated flow control valves to ensure a constant velocity as the pressure in the accumulator decays.

In your case, with no flow control valve in place, the velocity of the cylinder will be a function of the mass flow rate and the inside diameter of the tube. The mass flow rate is indeterminable, the accumulator will try to empty itself as quickly as possible and will only be slowed by the restrictions between the accumulator and the cylinder.

I would imagine that your valve will be actuated in less than 35 seconds. I don't know all the delta P values for you system are, but I would expect the cylinder to be very quick to start with and slower as the accumulator get closer to being empty.

Adrian