ASME B31.1 102.2.4 Ratings: Allowance for Variation From Normal Operation. 1

[curtis2004]

Hello Everyone,

I am trying to determine design pressure of penstock in hydro power plant. Currently there are mix of Class 300, 400, and 600 valves are used as drain valve for different equipment close to the bottom of the penstock. The elevation change from maximum elevation of reservoir where penstock starts to bottom of the turbine which discharges to a lake is 1,355'. This results in static head of 1,572:2.37 = 572 psig.
There was commissioned a Study which analyzed different emergency scenarios and associated pressure transients with it. Based on this Study, worse case scenario was emergency TIV (Turbine Inlet Valve) closure, during which pressure rises to 806 psig at the inlet of TIV. My first instinct was accept 806 psig as design pressure. However, later I realized that this is not normal sustained operations. Main concern here was we need to replace all Class 300 valves and flanges because MAWPat 38C for Class 300 is 740 psig.

Clause 101.2.2 Internal Design Pressure states "The internal design pressure shall be not less than the maximum sustained operating pressure(MSOP)within the piping system including the effects of static head."

There is a clause 102.2.4 Ratings: Allowance for Variation From Normal Operation, which states "The maximum internal pressure and temperature allowed shall include considerations for occasional loads and transients of pressure and temperature. It is recognized that variations in pressure and temperature inevitably occur, and therefore the piping system, except as limited by component standards referred to in para. 102.2.1 or by manufacturers of components
referred to in para. 102.2.2, shall be considered safe for occasional short operating periods at higher than design pressure or temperature. For such variations, either pressure or temperature, or both, may exceed the design values if the computed circumferential pressure stress does not exceed the maximum allowable stress from Mandatory Appendix A for the coincident temperature by
(A) 15% if the event duration occurs for no more than 8 hr at any one time and not more than 800 hr/yr, or
(B) 20% if the event duration occurs for not more than 1 hr at any one time and not more than 80 hr/yr".

I checked ASME B16.34 which only allows 10% variance from rated pressures and only in case of PRV, PSV, and Rupture Disc operations at clause 2.5.1. There is a Clause 2.5.2, which doesn't says anything except it is responsibility of the end user in case of other variances if valve is damaged by exceeding pressure/temperature rating.

There is Clause 2.5.3 System Hydrostatic Tests, which states "If valves conforming to this Standard are subjected to hydrostatic pressure testing of piping systems with the valve in the closed position at pressures greater than the 38°C (100°F) rating, or, if applicable, at pressure greater than the closed pressure differential shown on the valve identification plate (see para. 4.3.3), damage resulting from such testing is solely the responsibility of the user. In the open position, valves installed in a piping system may be subjected to system pressure tests at pressures that do not exceed the hydrostatic shell test of para. 7.1, provided the user has determined that there are no functional limitations, for example, restrictions on actuating devices or special materials of construction".

They even do not recommend include valves for hydrostatic tests at 1.5 x rated pressure.

I could only find similar discussions on B31.3 few years back:

https://www.eng-tips.com/viewthread.cfm?qid=369257

What do you think, is Class 300 valve per B16.34 is good for above mentioned conditions?

Thanks,
Curtis

 

[bcd]

You are referring to an older edition of B16.34. More current editions split paragraph 2.5.3 into 2.5.3.1 for valves in the closed position, and 2.5.3.2 for testing with the valves in the open position.

Shut-off pressure ratings of the valve internals can be less than the full pressure class rating of the valve body. So applying 1.5x the class rating across a closed valve can damage the internals (closure member, shaft, seat, etc.).

Per 2.5.3.2, it is permissible to pressure test an open valve in a system up to the Shell test pressure of paragraph 7.1 in B16.34.

So it is ok to test a valve complying to ASME B16.34 that is installed in a piping system up to 1.5x the 38 C (100 F) class rating for the body material.

 

[curtis2004]

Hi BCD,

Thanks for your answer and correction. Yes, I had older version (2004) of B16.34. I found newer version (2017) which is a bit different. I understand and appreciate that with gate valve open, piping system can be tested to 1.5 x the class rating.

It sounds like ASME B31.1 para 102.2.4 Ratings: Allowance for Variation From Normal Operation doesn't apply for valves build to ASME B16.34. It is because this is limited by component standard. Or is it not?

2.5.2 Other Variances. Damage that may result from subjecting a valve to other operating variances (transients) in excess of its pressure rating is solely the responsibility of the user.

What 2.5.2 supposed to mean?

If they are ok with 10% overpressure in limited cases with limited duration as per clause 2.5.1 Safety Valves, Relief Valves, or Rupture Disk Operation, why not apply the same for example, for pressure transient caused by emergency valve closure, which will last less then 1 minute?

Thanks,
Curtis

 

[bcd]

First, piping systems are not normally operated at the full class ratings, so the additional overpressure allowed by 31.3 is usually not an issue for valves per B16.34.

2.5.2 in B16.34 is clear that when users damage valves by subjecting them to higher than their rated pressure, the user is responsible. It is expected that a valve will survive pressure up to 10% the class rating for short duration to accommodate the accumulated pressure during safety valve relief.

Many valves have been damaged by negligence such as trapped fluid and thermal expansion that generates unknown pressure above the rating.

Design criteria for valves is different than that for pipe. Valve pressure boundary design is based on time dependent material properties (creep). Not yield strength. When valves are subjected to excessive pressure over longer durations, the body can distort, critical sealing dimensions lost resulting in leaks of hazardous media and possibly jam and prevent operation. This is the main reason why valve design and pressure criteria is different than the pipe codes.