20 replies to this topic

[RAM1975]

I am attaching the PFD of our's refrigeration plant in which all the pressures are shown.

 

The red indicated Duel safety relief valve is found poping at the pressure of 14-15 Kg/Cm2 A.

 

Where as the valve is calibrated and setted to operate at 21 Kg/CM2 .We really do not know what hydralic thurst come for seconds and the valve pops up.

 

The max. discharge pressure of compressor is set at 14.5 Kg/Cm2 .Hence there is no possibility that the pressure could go beyound this.

 

We wish to take the decision remove this safety valve and put dummy in place .But how can i make my safety dept.

 

This is ammonia refrigeration chiller.

 

The duel safety relief valve is located over the economiser .this is shell and tube type.

 

RAM

Attached Files

•  PFD 25.11.13.xls   930KB   55 downloads

Edited by RAM1975, 30 November 2013 - 05:06 AM.

[fallah]

RAM,

 

Lack of adequate info...

 

Please specify PSV type and submit the PSV and Economizer data sheets if it's possible...

 

PSV discharge line destination? Back pressure value? If it's variable or constant?

[curious_cat]

 

We wish to take the decision remove this safety valve and put dummy in place .

 

Has this change been approved? Sounds dangerous to me. I may be wrong. 

Edited by curious_cat, 30 November 2013 - 08:54 AM.

[S.AHMAD]

1. The receiver pressure is 14.5 kg/cm2.

2. The economizer is located at the bottom of the receiver and hence the economizer is full of liquid. Did you take into account of the liquid static head in your pressure setting?

[Ajay S. Satpute]

Hi,

 

Can you share the PSV datasheet? Several relevant data could be traced there (like back pressure, type, CDTP etc.)

 

Regards.

 

Ajay

[RAM1975]

Lot s of thanks to all you .

 

This is bought as Package equipement ,I have asked the supplier to send me the DATA sheet for PSV . I will load this as soon as i get this.

 

Curious Cat -we yet to take this decision ,Before we make this decision all the possiblities we are studying.

 

Ahemad -your point i feel is very valid since the reciever is located at 10 Mtr height on the terrace and the liquid line is coming directly down over the economiser .But the pressure indicator on the economiser never showes this pressure exceeded greater than 14-15 Kg/Cm2.

But we observed the safety valve pops for seconds and then shuts.Pressure indicator is unable to record this fractions of seconds.

[fallah]

RAM,

 

At least specify the PSVs type, their discharge line destination and relevant superimposed back pressure...

[Ajay S. Satpute]

Dea Ram,

 

I recall a similar problem attended recently. The liquid service PSV used to open and keep open at pressure very lower than the set point.

 

Existing system: Conventional PSV, Back pressure = 30 kg/cm2g, Set pressure = 34 kg/cm2g.

Due to process modifications, it was observed that back pressure is no longer 30 but 10 kg/cm2g.

 

Proposed system: For both the back pressure values (existing and proposed system), conventional PSV is not recommended  (>10% of set pressure criteria). Conventional type PSV was replaced with balanced bellow type of correct size and problem was solved.

 

Do you think something similar is happening in your system?

 

Regards.

 

Ajay S. Satpute

[fallah]

 

Proposed system: For both the back pressure values (existing and proposed system), conventional PSV is not recommended  (>10% of set pressure criteria).

 

Hi,

 

As long as superimposed backpressure is constant such criteria isn't required to be followed for conventional type PSV, because the spring load can be adjusted accordingly...

[Ajay S. Satpute]

Fallah,

 

We had checked spring range of the existing PSV, which was beyond our requirement.

[curious_cat]

 

The red indicated Duel safety relief valve is found poping at the pressure of 14-15 Kg/Cm2 A.

 

Where as the valve is calibrated and setted to operate at 21 Kg/CM2 .

I don't think the valve is popping at 15 kg / cm2. What I suspect is you have a Pressure transient that does hit 21 kg/cm2 but is too quick to register on your Pressure indicator but nevertheless enough to pop open your relief valve. 

 

If you had a Pressure transducer and an oscilloscope or something similar you might be able to capture this. 

 

 

The max. discharge pressure of compressor is set at 14.5 Kg/Cm2 .Hence there is no possibility that the pressure could go beyound this. 

 

I think the discharge P is a steady state pressure. There's no guarantee that a downstream disturbance or some other form of transient may not exceed this P especially in an upset. 

 

Again this is speculative in the absence of richer data. I may be wrong.  

 

What I'd try to find out is, at the moment the PSV pops what other correlated events happen, if any. Is it induced by some other identifiable event? A control valve throttling or compressor change of state etc. 

Edited by curious_cat, 02 December 2013 - 06:27 AM.

[fallah]

 

We had checked spring range of the existing PSV, which was beyond our requirement.

 

Hi,

 

I meant when you had set pressure/back pressure=34/30 kg/cm²g and hadn't premature PSV pop up...Otherwise for new back pressure you are right and the spring load can remotely be adjusted to change from 4 kg/cm²g differential pressure to 24 kg/cm²g one...

Edited by fallah, 02 December 2013 - 06:38 AM.

[fallah]

RAM,

 

If hydraulic thrust or pressure spike occurrence would be possible, not only cannot cause the PSV opens prematurely but also causes the PSV would be closed at the set pressure, especially if the PSV to be spring loaded type which is inherently "slow acting"...

 

Then, if the PSV is well specified and calibrated the most probable reason for such PSV malfunction may be the difference between actual back pressure and prespecified one...

[cesar cazarotto]

I have worked with a similar amonia system and I suggest you verify if the condenser has some inerts (e.g air / nitrogen) because the evaporators work under vaccum. In this case you can have high pressure in the accumulator vessel.

Another point is wrong set pressure of the PSVs, please check the set pressure.

[RAM1975]

The following are answer

 

Psv Type- The Make is DANFOSS PS -40

 

Discharge line destination- The discharge line is 10 mtr high open to atmosphere .

 

Relevent superimposed back pressure- As the line is open to atm.

 

Thanks to all you 

[fallah]

 

Psv Type- The Make is DANFOSS PS -40 

 

RAM,

 

I meant it is spring loaded or pilot operated..

 

Anyway, considering the PSV discharge line routed to atmosphere, it might relieving temperature of the PSV be much different than test pressure and temperature correction factor hadn't been applied in PSV test pressure. Please specify relieving temperature and CDTP of the PSV for above mentioned investigations...

Edited by fallah, 03 December 2013 - 01:29 PM.

[RAM1975]

Sir,

 

Sorry for the delayed response,I Checked everything writtten above the safety valve but no where i found any information for the CDTP.

 

The temp range for liquid ammonia is 10-12 degree centigrade and max. is 35- to 40 Degrees centigrade.

 

One thing which i do not understood , it is clearly written over the safety valve for use in GAS but i do not understood why vendor is used it in liquid service.

 

Above the safety valve it is clearly written the valve is set for 21 KG/CM2.

 

But when the safety valve is calibrated (here in Factory) it is done as per the present ambiant conditions that is at 24 to 26 degrees centigrade room temp.

And Nitrogen pressure is used for checking.

 

P.S.

 

The valve is spring loaded.

 

 

 

Ram.

Edited by RAM1975, 05 December 2013 - 06:34 AM.

[RAM1975]

Dear Fallah,

 

Request you to reply on the querry,

If any additional data is required i will add.

 

Ram

[fallah]

RAM,

 

The PSV has been specified to use in gas service while actually is used in liquid service, and this might be the key point for the valve malfunction...

 

Check out with vendor if the valve has dual certifications for both liquid and gas services. A liquid service valve normally can have dual certifications but a gas service valve cannot.

 

Let's know the checking result for further thoughts....

[S.AHMAD]

RAM

1. Since the releasing of ammonia from the PSV is not continuous bur few seconds (Is that correct?). This suggest that the pressure increase is caused by sudden surge in the system.

2. Therefore, one possibility due to rapid closing of any downstream valves (I saw a few in the diagram). Since the economizer is full of liquid, sudden closing of any downstream valve leads to sudden increase in the back pressure (few seconds). Unfortunately you don't have any record that shows the operating pressure when this happen.

[RAM1975]

Thanks for the suggestion i took the same answer with the supplier and then we undertook the resettings of the downstram valve and so adjusted that the rapid clousure should not happen .There is lot of improvement after this work.

 

But now under observation i am not sure if this is eliminated.

 

thanks to the forum and thanks to all helps i recieved.