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Reference Data
Continued..
D) Piping & Valve Considerations: The ANSI/ASME B31 piping code and its variations, as applied to high pressure steam boilers, provides specific guidelines as to procedures and methods for the Power Piping adjacent to the boiler, and related distribution piping. This piping code has been adopted by many states as it applies to installed vessels constructed to the ASME Sect.1 code. This same piping criteria may be applied to fuel system or feedwater treatment system piping at substantial additional expense. However, applying these code requirements to these "auxiliary" systems may not be of obvious benefit to the owner. In many applications utilizing factory packaged systems, an acceptable alternative is to provide the auxiliary system piping constructed in accordance with the manufacturers proven standard cataloged methods, using standard good industry practices and complying with applicable insurance codes.
Multiple high pressure steam boilers attached to a common header or steam distribution system require a non-return valve in addition to the main steam stop valve on each boiler. This valve has a disc that must be substantially open during predicted normal flow conditions to avoid "chattering" and the resulting excess wear that leads to valve failure. As such this non-return valve must be properly sized and will likely be smaller than the boiler outlet nozzle and attached steam piping. The B31 Power Piping code sets jurisdictional limits, and all piping and valves within those limits must be constructed and installed as if part of the ASME Sect.1 boiler itself. This typically requires an ASME "PP" or "S" stamp on the part of the installing contractor.
5) THERMAL SHOCK
Thermal shock is a leading cause of boiler damage. Thermal shock does
not respect a boiler's age, and can manifest itself in different ways.
While more prevalent in hot water boiler systems, thermal shock can
also occur in steam boiler systems. Note the reference to "systems." A
boiler by itself cannot suffer thermal shock without a system problem
or an operational error. For hot water boilers, rapid replacement of
the heated boiler vessel water with cold water from the distribution
system can cause tube sheet cracks and at the least, leaking of the
tube/tube sheet connection. In simple terms, this is caused by the
unequal contraction of the tubes as they cool, versus the hot tube
sheet. This condition may require simple re-rolling of tubes, or
substantial tube sheet repair or replacement.
Thermal shock is also induced in either a steam or a hot water boiler by firing a cold boiler at a medium to maximum firing rate. All boilers should be allowed to heat up to, or near, the operating temperature at the lowest firing rate, before the unit is placed in unrestricted operation. The stress due to this type of thermal shock can cause the hot (or rapidly heating) tube sheet to apply stress to the cold tubes in a firetube boiler or in areas adjacent to the radiant flame in cast iron boiler sections. Steam cast iron boilers can also be susceptible to thermal shock damage due to large cold water returns from condensate return systems or automatic water feeders due to a relatively small water content.
As applied to hot water systems, steel firetube and cast iron boilers can operate with inlet and outlet temperature differentials of up to 40.F, and perhaps a little higher with proper return condition control. Flexible steel water tube boilers are capable of inlet/outlet differentials of well over 100.F, and some have a 20 year warranty against thermal shock. The actual return limitation for these units is based on the fuels to be used. Steel inclined water tube boilers can have inlet outlet differentials of up to 70.F, as can copper finned tube boilers. However, while these types of boilers have their place, they are also the most inexpensively made (much like a direct fired heat exchanger), and a favorite of commercial boiler repair companies.
While you can provide a large measure of protection with the type of boiler, prevention of thermal shock is a function of the system design, coupled with a proper control scheme, and proper operational procedures.
6) FUEL
CONSIDERATIONS
For the sake of brevity, the following addresses liquid or
gaseous fossil fuels. Coal, wood and waste fuels are discussions by
themselves, so we will confine these comments to typical commercially
available fuels.
Natural gas provides the best solutions for pollution control, but, not surprisingly, at a higher cost. For hot water systems, natural gas allows the greatest differential in HW system water supply and return temperatures – with return temperatures as low as 130.F. This limitation is due to the dew point of the fuel, the point at which, in natural gas for instance, the typical 15% moisture content starts to condense out of the flue gas. Nominally, this happens in boilers with greater than 83% combustion efficiency. Natural gas does not have the radiant heat value of fuel oil, and as such does not have as high a combustion efficiency. However, this is typically offset by the lack of issues and costs related to delivery, distribution, maintenance and storage.
7)
REFRACTORY or WATER (its someone's money)?
While there are many variations in boiler designs from
manufacturer to manufacturer, it can be safely applied that less
refractory equals less operational cost, and less downtime. Boiler
surfaces that are 100% water cooled in a proper manner will not wear
out in the fashion of refractory, and will not have the recurring
expense of routine maintenance, and the logical cycle downtime and
major repair. Except for those manufacturers retaining and emphasizing
"Dryback" boiler designs from the 1950s, virtually all modern firetube
type boiler manufacturers offer what is commonly known as "Wetback"
design for firetube type boilers (scotch marine and firebox). Even in
the case of larger industrial D-Type water tube boilers, surfaces
subject to direct radiant flame have been 100% water cooled by the
leading manufacturers as a standard part of their boiler design for
some time. The acceptance and the prudence of the 100% water cooled
("Wetback") design is emphasized by the manufacturers building boilers
31,050 LB/STM/HR (900 BoHP) and larger, in that it is the design of
preference.
8)
FEEDWATER TREATMENT
In the same light of "we are what we eat," the water entering
a boiler impacts the life of the unit, and the life and performance of
the system in which it is installed. As hot water boiler systems are
generally closed systems (barring leaks or component problems) and are
easy to treat and maintain, this discussion addresses primarily steam
boiler systems.
In broad terms, untreated water contains constituents that will coat the boiler internals with a baked on (hard or soft) scale, precipitate out and collect as what is typically termed as "mud" as the steam leaves the boiler, or remain suspended as undissolved solids in the boiler water. These conditions, or combinations thereof, reduce efficiency, inhibit proper performance, or set the stage for vessel component failure. These conditions can be addressed with pretreatment equipment as necessary for the application (water softeners, demineralizers, filters, etc.), with manual boiler bottom blowdown and automatic / manual surface blowdown, or periodic opening and cleaning as required. Make up water pretreatment to boiler systems is always a good idea.
Water also contains large volumes on non-condensable gases, primarily Oxygen (O2) and Carbon Dioxide (CO2). In depth technical discussions as to the impact of these gases on the steam / condensate system are available for the asking. However, for the space we have here, let us broadly say that the boiler (tube) surfaces need to be protected from Oxygen pitting and ultimately failure. The steam / condensate needs to be protected from corrosion, and the process
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