CAPACITY OF A PRESSURE RELIEF VALVE FOR A STEAM DRUM 2

[Saver2008]

Hi!

I have a Waste Heat Boiler with a steam production of 50,000 kg/h of saturated steam. I´m choosing to install 2 PSVs on steam drum. Regarding flow capacity, I cannot find the support of ASME CODE where indicates me that, I found on PG-67 BOILER, PG-67.1, the following:

PG-67.1 Each boiler shall have at least one pressure
relief valve. Boilers with more than 500 ft2 (47 m2) of bare
tube water-heating surface shall have two or more pressure
relief valves. For a boiler with combined bare tube
and extended water-heating surface exceeding 500 ft2 (47
m2), two or more pressure relief valves are required only if
the maximum designed steaming capacity of the boiler
exceeds 4,000 lb/hr (1 800 kg/h). Electric boilers with
a power input more than 1,100 kW shall have two or
more pressure relief valves. Organic fluid vaporizer
generators require special consideration as given in
Part PVG.

Above helps me to support to put 2 PSVs but for capacity kg/h for each PSVs, I cannot find it or I´m having a hard time to determine that flow of steam for each PSV.

Do you have something in ASME that I have not looked for?

Thanks in advance.
RS

 

[The Obturator]

You need to first do a sizing calculation using the capacity of your boiler, to determine the (orifice) size of safety valves required - See PG-70 (ASME I 2021). Once you have selected the (orifice) size, you can then easily do a maxiumum capacity calculation of each valve.

As a guide to ASME I sizing - take a look at the Emerson (Crosby) Pressure-relief Valve Engineering Handbook (refer to page 5.6 onwards)

https://www.emerson.com/documents/a...-valve-engineering-handbook-en-gb-4244934.pdf

*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[georgeverghese]

Use a staggered setpoint arrangement:
a) A smaller PSV sized for say 25-30% of 50e3kg/hr set at 100% of design pressure, and with relieving pressure at 116% of process design pressure
b) A larger PSV sized for the remaining load, set at 105% of process design pressure, and relieving pressure at 116% of process design pressure

ASME allows a relieving pressure of up to 116% of MAWP for staggered PSV arrangements. This arrangement is valid for ASME compliant systems. For European systems, relieving pressure is limited to 110% of process design pressure.

Most operational reliefs will be small, and require only the smaller PSV to be activated. This way, if you do need to remove a leaky PSV (because it has lifted), you need only remove the smaller PSV. This also minimises the reaction forces on inlet and exit piping.

 

[The Obturator]

@georgeverghese what you are describing applies only to ASME VIII. There are different rules for staggered set pressures for ASME I which is the subject of the OP.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[chicopee]

Safety valves will always have on their bodies labels stating the relieving capacities and set pressure as long as they are NB/ASME labelled in the USA. Check the catalogs of the manufacturers of these valves or talk to the distributors of the valves

 

[georgeverghese]

See chat on this thread for ASME I rules re staggered settings on PSVs': Rules appear to be
a) One PSV may be set at MAWP, while the second may be set at 103% of MAWP
b) For both PSV's, relieving pressure must not be higher than 106 % of MAWP
See
thread1203-196378

 

[IM383]

To the OP, Saver2008, regarding the relieving capacity of your safeties, your answer isn't far from where you are looking. I am a commissioned Boiler & Pressure Vessel Inspector through the National Board of Boiler & Pressure Vessels Inspectors with 20 years of experience and hold Inspector commissions in multiple jurisdictions. I actually created an account here just to answer your question while perusing the boards for something else. I hope the following helps and provides the guidance you need.

If you take a look at 2021 ASME Section 1, PG-67.2.1.2, "[highlight #FCE94F]The minimum required relieving capacity in pounds per hour of the safety or safety relief valves on a waste heat boiler SHALL be determined by the Manufacturer.[/highlight] When auxiliary firing is to be used in combination with waste heat recovery, the maximum output shall include the effect of such firing in the total required capacity. When auxiliary firing is to be used in place of waste heat recovery, the required relieving capacity shall be based on auxiliary firing or waste heat recovery, whichever is higher."

Assuming that you don't have an auxiliary firing method and this is purely a waste heat boiler, the answer is simply that the total relieving capacity needs to be determined and provided by the
Manufacturer. The capacity or flow must be sufficient enough to prevent the boiler pressure from exceeding 6 percent above the MAWP (maximum allowable working pressure).

For all other boiler types there is a simple method of calculating the square feet of heating surface by a factor related to the boiler type (Water-Tube, Fire-Tube, Coil), the fuel type, and method of firing. However, that calculation does not work for waste heat applications and gives you a number far in excess of what is needed.

Regarding your set pressures, NBIC Part 4 Section 2.2.5 and ASME Section 1, PG-67.3 are almost exact copies of each other as written. Essentially, when two or more relief valves are used, the highest pressure setting shall not exceed MAWP by more the 3 percent. One of the valves must be set at or below the MAWP. The complete range of pressure settings of all pressure relief valves on a boiler shall not exceed 10 percent of the highest pressure to which any valve is set. The only exception to this is on a high temperature water boiler.

I know this is more than what you asked. The simple answer is that you yourself, nor I, can or should calculate the required minimum relieving capacity of a waste heat boiler. There are a multitude of considerations regarding design, operation, and construction that need to be taken into account. The code requires that the Manufacturer determine and provide you with the total. I would start with them. I hope this helps.