Page 320 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 305

scheduled by the boiler and the load more than anything designed to handle flashing steam. There’s supposed
else so the operators have to get at them quickly. to be an orifice or manually adjusted throttling valve to
take the pressure drop located in the piping close to, and
Valves, Feedwater farther from, the boiler than that automatic valve. If not,
On low pressure boilers the code requires a stop or the orifice is removed or the throttling valve opened
valve and check valve on the boiler feed piping but the wide the automatic valve will most certainly fail.
pipe itself isn’t controlled by the code. The Code has Bottom blowoff valves come in a variety of forms
specific requirements for the feedwater piping on a high but the most important part of their construction is they
pressure boiler out to the second stop valve and also re- don’t have any pockets or cavities that sludge can settle
quires a check valve. That arrangement normally means into and plug up. That’s the idea anyway. I can only
the bypass valve and isolating valve for the feedwater remember one time where I was tearing valves off to
control valve are both within the limits of the boiler ex- unplug a line and that’s because someone had installed
ternal piping. the valves backwards so all the mud settled on top of
The shutoff valve is there to allow you to isolate the discs preventing opening the valves. There are two
the boiler from the feedwater supply when it’s shut things that are stressed by these points, use proper
down and, more importantly, isolate the feedwater valves (ones designed for bottom blowoff applications)
system from pressure in the boiler. The check valve is and make sure you installed them in the right direction.
there to help prevent draining water from the boiler in See the section on normal operation for operation of
the event there’s a failure of the feedwater supply and, blowoff valves.
more importantly, preventing boiler water leaking out to The code for high pressure boilers requires two
produce a steam explosion if there’s a failure of the pip- valves for bottom blowoff designed for the service. The
ing. Unlike the steam valves and piping there is no code piping connecting them and the boiler must be at least
requirement for a free blow drain connected between schedule 80 (extra heavy) of materials certified to com-
the two isolating valves on a high pressure feedwater ply with ASME Codes and all welded. Piping inside the
line; there should be, and for the same reasons. second valve must be fabricated by a manufacturer or
contractor certified by ASME to do that work (See the
Valves, Blowdown and Blowoff section on ASME Code construction). All other blowoff
The valve for manual control of continuous blow- and blowdown connections only require one valve and
down (surface blowoff) should be provided with an the code piping requirements are limited to the portion
indicator that shows the position of the valve so an between valve and boiler. You may find two valves in
operator can restore a particular valve position. Some other lines because the owner or contractor elected to
valves are fitted with indicators and tapered throttling have an accessible valve to use with the code required
guides as part of the disc so the flow rate through the valve and piping located close to the boiler thereby lim-
valve is proportional to the indicated valve position. The iting the extent of the code piping.
ability to closely regulate the flow of blowdown (inde- Low pressure boilers and some high pressure boil-
pendent of automatic blowdown controls) permits the ers are equipped with quick opening valves, a valve that
operator to closely control the concentration of solids in works something like a gate valve but has a steel plate with
the boiler. a hole in it that is positioned in line with the pipe so there’s
It isn’t essential and not required by the code but no way for mud to plug the valve. The code permits one
I would strongly recommend installation of a check of the two valves required on high pressure boilers to be
valve between the continuous blowdown control valve a quick opening valve. There are rules for operating those
and the boiler. Should you forget to close the continu- valves (see normal operation) and they should be installed
ous blowdown valve it will prevent water from another in a manner that makes it easy to use them.
boiler entering the idle boiler. It’s also like using a non- The seatless blowoff valve (Figure 10-39) is a com-
return valve, if you chose to you can rely on the check monly used bottom blowoff valve and one that I have
valve so you don’t have to close the blowdown control seen operated improperly more than any other valve
valve (and reset it later) for short outages. (see normal operation) but it is easily repaired. Unlike
I’ve seen many an automatic blowdown control other types of valves it doesn’t require skill or special
system isolated because the blowdown control valve tools to repair or even adjust to restore its shutoff ca-
failed. On most small boilers these are quarter turn pability. The manufacturer’s instruction manual is very
motor actuated valves or solenoid valves which aren’t important reading before working on these valves.