Implications of Level indication in ASME Section 1 for Power Boiler Drums

[sydneygarrett]

I am seeking information about ASME Section 1 for Power boilers. I have been told that it says the code requires each fired boiler drum to have a gauge site glass and requires two (2) gauge site glasses for all boilers above 400 PSI.

My question is: does the code say / imply that the boilers must have site glasses at minimum, but another form of level indication would suffice? Or would any different form of level indication be in addition to these site glass indicators?

Example: If the boiler is operating above 400 PSI, would a magnetic level indicator be OK to replace one of the site glass gauges, or would there still have to be 2 gauge site glasses, and then the magnetic level indicator would have to be additional?

I'd love to be able to reference the wording of the code, but I don't have access to it.

 

[metengr]

Here is what the 2013 Edition of Section I states;

PG-60.1.1 Boilers having a maximum allowable
working pressure of 400 psi (3 MPa) or less shall have
at least one gage glass in service at all times. Boilers having
a maximum allowable working pressure exceeding
400 psi (3 MPa) shall have two gage glasses. Instead of
one of the two required gage glasses, two independent remote
water level indicators (two discrete systems that
continuously measure, transmit, and display water level)
may be provided and the required gage glass may be shut
off, but shall be maintained in serviceable condition.
Electrode‐type electric boilers are required to have only
one gage glass, regardless of MAWP.
Boilers not having a fixed water level, such as forcedflow
steam generators and high‐temperature water boilers
of the forced circulation type, are not required to have
a gage glass.
PG-60.1.1.1

 

[timsch]

So if I read this section of the code correctly, under no circumstance can an alternative style level gage (i.e. Magnetic Level Gauge or Guided Wave Radar) replace every Glass Sight Gage, shut off or not, on a fixed water level Boiler. Please let me know if this is not correct.

Is it typical to have a fluid level gauge on a Boiler that does not have a fixed water level, even though a glass gage is not required?

 

[sydneygarrett]

@metengr - Thank you, this is helpful.

@timsh - Do you think that an alternative style gauge could replace ONE of the sight glass gauges for one of the two required above 400 psi?

 

[timsch]

The code states that the replacements must continually measure, transmit, and display the level, so it looks as though a Magnetic Level Gage alone would not suffice. If it were paired with a radar or something else that would connect to a transmitter, then that would be acceptable per this section of the code.

A Magnetic stand-alone gage could be used for non-fixed water level boiler.

I'm curious as to why the insistence of a glass gage. Would it be due to a lack of confidence in a gage that does not provide for the visibility of the fluid itself?

 

[csr17]

metengr posted the correct code reference.

Every boiler above 400 psi *must* have two gauge glasses. One of them may be replaced by two independent remote water level indicators - such as probe systems or DP transmitters. Under this condition, the other gauge glass may be valved out of service, but it must remain in working condition (i.e. the operators must be able to valve it back into service and have it working properly if necessary). So, in practical terms, the boiler can operate without a gauge glass, but there must be a gauge glass installed (i.e. connected to drum taps) and it must be in working condition. Unfortunately, this is often done incorrectly.

Many end users don't like working with gauge glasses for a number of reasons: (1) there can be dangerous (potentially fatal) steam leaks if the glass fails; (2) the level may not be visible over time due to glass erosion, sediment buildup, etc; (3) the steam drum is often in an inconvenient location (high, warm) for operators to physically go look at the glass; (4) the glass illuminators (if required) don't work properly. Glass failure is highly dependent on operating pressure and regular maintenance patterns. At a pressure as low as 400 psi, a gauge glass from a reputable supplier should need minimal maintenance and should work just fine. Old illuminators had incandescent lightbulbs, which were a pain because they would get hot, burn out, etc. but modern illuminators have red/green LED's and they're much better.

The code rules on gauge glasses have softened over the years, and many end users use the option mentioned above and valve the gauges out of service (whether they do it correctly or not is another story), but it is likely that gauge glasses will never go away. It is the only "direct" method of knowing the water level in the boiler. Controlling the water level is very important. Incorrect water level can range from turbine damage (high level) to boiler damage (low level) and can be very dangerous.

With regards to magnetic level gauges - no they can't replace gauge glasses. Also, magnetic gauges aren't allowed at all above a pressure of 900 psi. Most magnetic gauge chambers are made of stainless steel, and the code is strict about using stainless steel on boiler drums because of stress corrosion cracking. In my experience, magnetic gauges have mostly been used on boiler blowdown tanks. A regular sight glass should be used on the drum.

There is more information in PG-60 of Section I. Check it out if you can.

May I ask where this question is coming from? Do you work for an end user?