5 replies to this topic

[ChemiFreak]

Dear all,

 

API STD 521 sec. 4.3.4.4, regarding the check valve flow area to be considered for reverse flow scenario of PRV sizing, states "one  may  estimate  the  reverse  flow  through  series check valves as the flow through a single orifice with a diameter equal to one-tenth of the largest check valve’s nominal  flow diameter.", while DEP 80.45.10.11-Gen sec. 2.11.1 point 5 states "Where there are dual Class 1 check valves, calculate the backflow assuming a check valve flow area of 10 % for new facilities."

 

10% of area and 10% of diameter do give significantly different PRV orifice sizes.

 

Could someone enlighten me why the two standards differ and which one would be more appropriate.

[ankur2061]

ChemFreak,

 

Any design tandards, design guidelines and design engineering practices are there to get a design engineer started on the basics. What would be applicable for one  client  / plant may not be applicable to another client / plant.

 

Hence it is clear that clients who follow or accept "Shell" design engineering practices for engineering design would prefer to go what the DEP prescribes. However, if a client is not inclined to accept "Shell" DEPs or insists on something more acceptable on an international scale such as an API standards, then you would have no other option but to proceed accordingly.

 

The beauty of engineering design is that you can have multiple approaches to design based on a combination of the client's requirement, regulations / codes / standards applicable to the geographical location where the design will be used and the specific type of process technology being utilized.

 

I hope this gives you some idea of how to proceed.

 

Regards,

Ankur.

[ChemiFreak]

Thanks Ankur.

 

But I just wanted to know whether API guideline of flow through 10% of diameter would be sufficient enough (because of the smaller orifice size) compared to 10% of flow area as per DEP at similar circumstances.

[fallah]

ChemiFreak,

 

The main point is the base of calculation in two references: In the API the nominal flow diameter of the largest check valve is as the base for orifice diameter calculation while in DEP the only check valve flow area is to be considered as a base for calculation; and these various bases leading to the results to be close. Nevertheless it's obvious that the DEP method is more conservative than that of the API but i think the API criteria would be sufficient to do subsequent calculation for relevant relief load...

Edited by fallah, 22 August 2014 - 05:22 AM.

[lokeshmiddha]

My understanding of DEP 2010 edition is

 

" there are two options to avoid back flow

 

a) provide either a single class 1 check valve with the upstream relief device sized for check valve back flow. Sizing basis for PSV for back flow relief case is  calculate the backflow assuming a check valve flow area of 10 %.

 

Provide dual class 1 check valve

 

And as Per API, if single check valve is there then assumed complete failure and for dual check valve calculate reverse  flow  through  series check valves as the flow through a single orifice with a diameter equal to one-tenth of the largest check valve’s nominal  flow diameter.

 

based on this i believe API is more conservative.

 

There may be changes after 2010 edition of DEP.

[Bobby Strain]

Shell is conservative in their designs. Sloppy in their operations.

 

Bobby