Steam boiler Steam production vs safety relief valve capacities 1

[Jedediah]

We are looking to raise throughput and I am evaluating the relief valves on the process unit to ensure we are adequately protected. One of the items I was reviewing was the steam production from a Steam Drum. The nameplate shows a steam production rate of 20,842 lb/hr. The relief valves on the steam drum are good for 31,148 lb/hr. My question is whether the nameplate steam production limits actual steam production to that value or is that simply a guidance for what the unit was designed for. It seems that the real limit should be our installed relief valve capacity.

Appreciate the response.

 

[The Obturator]

If you are getting the safety valve capacity of 31,148 lb/hr from the valves nameplate, be aware that this is most probably the 'Certified Capacity' at the stated set pressure, but this will be for saturated steam.

Its best to get the manufacturer to run a capacity calculation against actual conditions, including the effect of superheated steam. Best to get all the safety valves for your boiler re-evaluated for size, since ASME I dictates 75% of boiler capacity through drum valves etc.

(Note there is also a separate safety relief valve forum).


*** 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]

You may run into several throughput bottlenecks as you try to raise steam production capacity, especially when you contemplate a 50% increase. Most onerous of which may be that burners will have to fire harder ( assuming burners have sufficient excess firing capacity) and that will increase tube metal wall temps in the radiant section of the boiler to beyond what it was intended for, thus reducing the expected life span of the boiler. Check the design calcs for the mechanical design of the boiler to find the mean metal wall temp of the radiant section tubes when running at 21e3 lb/hr, and how far that is away from the mechanical design metal wall temp.
Unless your boiler package and BFW treatment package designers were unusually generous in their offers to your Co, would suspect you wouldnt get more than 10-15%.

 

[Dhurjati Sen]

Check the superheater safety valve rating first, as that would be less than the drum safety valve rating and govern the excess steam generation.

DHURJATI SEN
Kolkata, India

 

[davefitz]

In addition to the metal temperature limits listed by georgeeverghese, there are other physical and legal limits you would need to address. These include:

a)The boiler feedpump is required to be able to supply the max steam generation capability when the steam drum is at lifting pressure plus permitted overpressure; if you increase steam capability the pump might not meet this requirement.

b)The fans that support combustion ( FD , ID and add'l fans related to the air pollution control equipment) are sized for the original nameplate rating, and also the related pressure or draft that boiler casing is rated for will vary by the square of the fluegas flowrate.

c) In most countries the plant's insurance policy will not cover informal uprating of the steaming capacity; you will likely need to have the unit officially rerated to maintain insurance coverage.

d)Likewise the stack emmissions are limited according to the site's environmental permit and increase in steaming capacity will cause the unit to exceed the permitted emission rate.

e) the water treatement plant might not be sized for the increase in capacity.


"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick