steam safety valve back pressure limit 1

[Safone]

This thread goes into much detail on how to calculate the back pressure on a ASME Section I safety valve: thread408-63173

But the basic question is: what is the allowable back pressure on such a valve?

I have scoured the codes and some vendor catalogs for guidance. The only thing I find is the time-honored "10% of set pressure". I know this is the typical value to prevent capacity/stability problems in conventional SV's, but I'm not convinced it is appropriate for ASME Section I. Read on...

Does anyone know of other guidance, or can you explain why 10% is OK given that the capacity is based on 6% accumulation. Per API RP 520, the back P limit for non-balanced valves appears to be related (directly) to the overpressure ( = accumulation when Pset = MAWP), hence my reasoning that 6% should be the limit.

 

[sterl]

See Crosby Engineering manual from Tyco Valves (large PDF) chapter 7.

Also see Chem Eng I believe 1969 articles from Anderson Greenwood employee, I do not have a copy immediately available but Google shoudl be able to track it down...

Short Version: At 10% SUPERIMPOSED pressure the SECTION 1 valve has a small decay in flow but is stably open...At something over 15% Superimposed Pressure flow their stability is challenged (they will pop open and slam shut) depending on how much of the SIB is a consequence of their own opening, and how much is a function of the downstream pipe's characteristics...

 

[EGT01]

ASME Section I is one area I have little or no experience so for my own benefit, I am interested to see how this thread goes. I have done some checking and like Safone, I'm not finding the same level of direction for sizing inlet and outlet piping for Section I relief valves like you do for Section VIII.

Sterl, the Crosby Handbook is a good reference for Section VIII systems but the handbook specifically states (see chapter 1, introduction) it is NOT to be used for Section I systems. So it may be of limited use for us here. Also, I have a copy of some fairly recent Anderson Greenwood seminar material that includes a chart showing the effects of back pressure on conventional spring operated relief valves but my chart refers to "built-up" backpressure rather than "superimposed". It does indicate the same problems of capacity decay as the backpressure builds, with the valve closing when the built-up backpressure hits 15%, but it is based on a valve inlet pressure of 110% of set pressure. Again, I think this is referring to a Section VIII valve but maybe there is other info in the article that could shed light on Section I valves.

Spence Engineering has a handbook that gives some info about Section I valves and has a table that gives a side-by-side comparison of Section I to Section VIII. Unfortunately, there is not much there about pipe sizing. If you are interested, look for their "Basics of Relief Valves" shortcut at the following address...

http://www.spenceengineering.com/handbook/Index2.htm

Safone, ASME Section I does allow for 6% accumulation but what I'm seeing in the vendor catalogs is that the valves are certified to 3% overpressure. I believe this is done to satisfy the Section I requirement that the valve would attain full lift at no more than than 3% overpressure. Just something to keep in mind when talking about the "stamped" capacity of the valve.

Without hard reference otherwise, I tend to follow the rule that the built-up backpressure should be no greater than the amount of overpressure used to determine the valve's capacity. In other words, if you determine the valve capacity at 6% overpressure, then the built-up backpressure at the valve's capacity should not exceed 6% of set pressure for a conventional valve. This would ensure the resulting forces at the valve outlet were not greater than at the inlet and the valve should stay open and without affecting capacity.

The only other thing I found was in Consolidated's SV-1 manual (try this link)....

http://www.consolidatedvalve.com/internet/pages/documentwarehouse/documentsearchresults.cfm

otherwise, use the following and click on the Document Warehouse and then search for SV-1.

http://www.consolidatedvalve.com/

The Consolidated SV-1 manual actually shows some examples in the General Information section (How to order a 1511 Safety Valve, How to order a 1541/1543 Safety Valve) that, for a Section I valve
Set pressure = 100 psig
Built-up back pressure = 5 psig
Allowable overpressure = 3%
but I would defintely suggest you discuss the backpressure issue with Consolidated before accepting that approach. I would be interested to hear what they had to say.

 

[Safone]

Sterl / EGT01

Thanks for the input. I'll advise once I've followed up your suggestions a bit more.

 

[JAlton]

Some thoughts on this issue. ASME Sec. I, Power Boiler, Safety Valves must discharge to Atmosphere. Back Pressure should not be an issue. Especially, Superimposed Back Pressure. There may be instances where the length of piping might cause built up back pressure, but by Code, the oultlet piping may not be reduced and should be as short as possible. Unless you are dealing with an Organic Vapor Generator in ASME Sec. I, Part PVG, or a High Temperature Water Boiler, you should not have issues with Back Pressure. ASME Sec. I, Power Boiler safety valves are designed to attain full lift by 3% overpressure and they are also designed with open yokes and exposed springs as well as open levers. There should not be a Back Pressure problem since the valve is not designed for discharge into a closed system. If you are dealing with a SRV on Organic Vapor or a HTWB, in both instances, the SRV must be designed for discharge into a closed system. If this is the case, then you should probably use the guidelines of ASME Sec. VIII, Div. 1.

J. Alton Cox

http://www.delucatest.com

 

[Safone]

EGT01:

The Consolidated catalog itself answered the basic question: the back pressure limits were quoted for different valve designs. What I found intersting is that the allowed limit was not constant, it varied quite a bit. E.G. the 1511 model valve can see up to 20% of Pset backpressure; others as high as 25%, but many at 10%.

I have not spoken to the vendors directly yet. I am assumng that these backpressure limits refer to flowing backpressure, but perhaps not????

 

[EGT01]

Safone,

Be sure you are not confusing the back pressure limits for mechanical reasons (pressure containment) on the discharge side of the relief valve with the back pressure limits for assessing valve flow capacity. You may want to clarify that with the vendor.

In general, the Section I relief valves being an open bonnet (open yoke) design, i.e. not totally enclosed design, may not be able to tolerate much back pressure from a pressure containment perspective. That may be what the back pressure limit is referring to in the Consolidated catalog.

Unless there is something about the valve design that makes it like a balanced bellows or piston type, I don't think I would size the relief system for a back pressure greater than the amount of overpressure.

If you talk to a vendor, let us know what you find out.

 

[JimCasey]

Section 1 valves have open bonnets. ANy backpressure will vent through the spring and likely constitute a hazard, or at least a mess.

Section VIII valves frequently have closed bonnets. Constant backpressure (example 100 psi set with 20 psi backpressure)superimposes its value upon the set pressure, but can be accomodated by setting the valve for, in this example, 80 psi; the differential between set and backpressure.

Safety-relief (SecVIII) valves expecting to see VARIABLE back-pressure would be specified with a bellows. The bellows effective area is the same as the disc, and the bellows is vented to atmosphere. Therefore, variation in back pressure does not superimpose on the set pressure. The valve sees the difference between the process pressure and atmosphere at all times.