Relief valve rating / capacity 1

[JimD1]

I have two 70-klb/hr boilers running at 125 psi. The management team wants drop the site operating pressure down to 90 psi.

I was informed that I need to add relief valves to the boilers due to the decreased operating pressure in the case of an 'event'.

This doesn't make sense. Can someone please explain.

Thanks, Jim

 

[normet]

Jim,
Code requirements require pressure relief on boilers so there should be relief valves present already. These could be left at there current setting as they are to be set relative to the maximum operating pressure of the boiler and not at the actual operating pressure which must be equal to or less than the the rated maximum operating pressure.

Relief valves need to be serviced on a regular interval by knowledgeable experienced personnel. Depending on your jurisdiction requirements, relief valve servicing firms may require some kind of jurisdictional certification.

 

[JimD1]

Thank you for your reply. We have 3 relief valves, services and calibrated annually offset by 3 psi each set below the max operating pressure of the boiler.

I was told that if we drop the normal operating pressure there would be more water in the boiler to flash into steam if there was event; therefore we needed addition relief valve to handle the increased amount of steam.

Jim

 

[MisterDonut]

If the relief valve setting are not changing, then this does not make sense. If the relief valve setting are supposed to be reduced, then the relief valves will need to be changed out and possible added to, as the size of the relief valve increases as the pressure decreases due to the increase in specific volume.
Use caution when reducing operating pressure. The boiler manufacturer designed the steam drum, separators and outlet nozzle for a certain operating pressure. Reducing the operating pressure will limit the amount of steam that can be generated without experiencing water carryover.

 

[puigi]

JimD1:

The difference in the density of the boiler water between 90 and 125 psi is minimal, look at the steam tables. Why is the operating pressure being reduced? Was the boiler or a down-stream component re-rated to a lower MAWP?
I would recommend against lowering the set pressure on the existing PRVs. The PRV's rated flow rate is specific to the design operating range of the spring and the ID of the nozzle. Lowering the set-pressure on the PRVs may leave you with insufficient relieving capacity.

PS: Lowering operating pressure reduces boiler efficiency.

 

[JimD1]

Puigi, Thanks.

There was an engineering decision made to lower the site operating pressure from 125 to 90 since most of the buildings operate with reducing valves and the highest building operating pressure is 60psi.

I was concerned that if we had a boiler 'event' that the relief valves may not operate properly.

Jim

 

[Duwe6]

Further, you might need new PRV bodies if you drop the settings to 90 psig. The required relief rate is 'set' by the total heating area of the boiler, and is a constant.

To change from 125 to 90 requires a new spring, and a bigger orifice to be installed in the PRVs. That bigger orifice is needed to get the same number of lbs/hour thru the PRVs at 90 psig as there was at 125 psig. Your existing valve bodies may not be able to accomodate the new orifices.

 

[JimD1]

OK, I just came out of a meeting. They want to drop the site operating pressure rom 125 psi to 90 psi steam. We are leaving the existing three relief valves set in the neighborhood of 160 psi each offset by 3 psi, we are going to operate at a new lower calculated max firing rate.

The boiler water level will remain the same. Does anyone see any issues, our chief engineer has an issue with the reduced operating pressure and the existing relief valves.

 

[MisterDonut]

I don't think you have an issue here. The relief valves are there to protect the boiler. They will not open unless the pressure in the boiler reaches the set point of the relief valve. Nothing that effects the sizing of the relief valves is changing.

 

[Duwe6]

Concur with MisterDonut. Your chief engineer needs to 'get over it'. If the PRV set was OK last month, the set is still OK tomorrow with the reduced rate. PRV's are to keep the boiler from exploding, and they are not to be used for boiler control. Once the engineer quits thinking of them as a control device, and considers them as just a Safety device, it will make sense.

 

[davefitz]

As far as protecting the boiler from overpressure failure of teh steam drum ,then the orignal relief valves should serve that single purpose. If the combustion system were to be modified to allow a higher heat input , then the boiler nameplate rating would change and so larger relief valves would be needed.

If the steam pressure setpoint were to be lowered, then a stoppage or trip of the steam demand from downstream systems would imply the boiler would have zero steam flow in the time period that the pressure increases to the relief valve opens. If the boiler has no overpressure vent valve or bypass system and does not have a feedforward MFT to adddress this scenario, then lowering the installed releif valve set pressure might help- would probably require new springs - the orignal flow arting would only be met at the original design pressure . Read section I for other details.

 

[Keith53]

Your engineer and you too are missing something.

In reality, operating at a pressure lower than design causes LESS water in the boiler, NOT more.....Are you confused yet?

I'll explain, but first, let's establish some facts;

A. Your boiler will NOT be producing less steam, just steam at a lower pressure. The loads will remain the same. If the buildings presently consume 50,000 lb/hr, they will continute to consume 50,000 lb/hr after drum pressure is reduced-therefore the boiler will STILL be evaporating the same ammount of water lb/hr.

B. You water level will be maintained at the exact level it is at now in the sight glass.

Bare with me cause in a minute you will understand why I stated those obvious facts.

Ok, so if you are going to be maintaining the exact same level in the sight glass, and operating at the same rate of steam production, why will the boiler contain less water when operated at a lower pressure? It's because at lower pressure the millions of steam bubble being generated in the boiler will be LARGER. Believe it or not, the boiler will be producing ruffly the same number of steam bubbles, but they will be LARGER due to the LOWER pressure in the drum. So, if those millions of bubbles are larger, then they DISPLACE more water, huh? ...And you stated that the relief valves are set at 160, so that means the boiler is designed and stamped to operate probably at 150 MAWP, right?

....If it is a 150MAWP boiler, then operating it at 90 PSI is almost 50% of it's design, right? Therefore it will be generating much larger steam bubbles and there will be a lot less actual water in the boiler than intended...Please remember that the size of the bubbles are critical because they displace water, and water is what cools the boiler tubes....Is the light starting to come on yet?

SECONDLY, seeing as how the buildings will STILL require the same amount of steam, it will have to be supplied at a higher velocity exiting the drum and thru the distribution header...Take care here because there are strict design limits to be considered before you make such a change. Operating at a lower pressure at higher velocities can cause pipe errosion and overload your condensate return system. It can also cause carryover and greatly lower the quality of the steam.

How do I know these things? We here just went thru all this and once I researched it, we had to back up and reconsider.

I KNOW why your engineer wants to lower header pressure. He wants to save money and he heard/read somewhere that lowering operating pressure has an immediate and direct payback...Well, he needs to learn to not believe everything he reads.

I suggest you research all I just stated above, do some calculations, then discuss it with your engineer, and hopefully you guys will enter into this in an experimental fashion where after you lower pressure, you will closely observe total loop operations to make sure all seems to be operating within its design abilities.

The relief valves? Forget them. Leave them alone and focus now on where the real problems might occur.

 

[Keith53]

JimD1,

In my above post I described what actually happens when you lower drum/header pressure much lower than boiler design.

Once we here discovered that, we compromised and it is working out well.

On our campus, steam loads are reduced to half once the workday ends. So at the end of the workday, we simply lower the control setpoint from 100 to 80 PSI, then an hour before the workday starts, we raise it back to 100 PSI.

Seeing as how on nights and weekends/holidays, there is MUCH less steam demand, we are not concerned with overheating the tubes, overloading the condensate loop, or increasing steam velocity...And yes, we have realized some cost savings.

 

[Keith53]

Oooops, forgive me, in my first post I meant to state your boiler is probably stamped at 160 MAWP, because you valves are set there.....I said 150 by mistake.