Boiler valve sizing

Boiler Steam Side Overpressure - All fired boilers are normally provided with PR valves sized to relieve the full steam rate in the event of closure of the normal outlet. 

Table 27.11 Hot water heating boilers - recommended sizes of relief valves...

Boiler or water-heating (kW) Cistern size (1) Ball-valve size (mm) Cold-feed size (mm) Open-vent size (mm) Overflow size (mm)... 

All reactors should meet or exceed ASME Boiler and Pressure Vessel Codes with respect to appropriate materials of construction. The unit should also be sized properly and equipped with safety relief valves and/or rupture discs and contain appropriate piping and controls. 

Before selecting the size of a steam-control valve, a supply main or even a steam boiler it is necessary to know at least approximately (or better) how much steam is to be supplied. Where steam is to be used as the energy source for heat exchangers this is the same as knowing the heat load that is to be met. 

Starting with the boiler(s), these will be set out giving due consideration to space between boilers and other items of plant in order to give adequate access to all equipment, valves and controls for operation and maintenance. For small- to medium-sized boilers 1 meter may be considered a reasonable space between items of plant where access is required. With large boilers (including watertube), this may be increased up to 3 m. The width of firing aisle will be dependent upon the size and type of boiler. 

Feedwater pipework will normally be gravity head suction from the hotwell or feed tanks to the pumps and at a pressure in excess of the boiler working pressure from the pumps to the boiler. Few problems occur on the pipework between pumps and boilers. However, inadequately sized suction lines can give rise to cavitation at the feed pump with subsequent boiler shutdown. The feed tank should always be positioned to suit the temperature of the feed-water and the pipework sized to give free flow at that head, taking account of bends, valves and filters. 

The design, size and siting of the boiler feed tank or hotwell must be compatible with the boiler duty capacity and system temperatures. They should be installed giving sufficient space for access to controls, valves and manways. 

Blowdown from the boiler(s) should always be taken to either a blowdown sump or blowdown vessel before discharging into drains. Both should be adequately sized to give cooling by dilution and be fitted with vent pipes to dissipate pressure safely. The boiler(s) should have independent drain lines for the main manually operated blowdown valve and the drains from a continuous blowdown system. Where more than one boiler is connected to either system the line should be fitted with a check or secondary valve capable of being locked. 

Note The above sizes apply to boilers fired with solid fuel. For oil-and gas-fired boilers the relief valve should be one size larger. 

The run and regeneration cycle on small to medium-size units is normally governed by a multi-port valve, which is the only moving part required. There have been many cases where this has failed and brine was injected directly into the boiler. A conductivity meter on the make-up line would guard against this. 

Codes and standards relevant to safety relief valves (SRVs) can vary quite considerably in format around the world, and many are sections within codes relevant to boilers or pressure-containing vessels. Some will only oudine performance requirements, tolerances and essential constructional detail, but give no guidance on dimensions, orifice sizes and so forth. Others will be related to installation and application. It is quite common within many markets to use several codes in conjunction with one another and it is not uncommon that specifications call for sections taken from several codes, which makes compliance by manufacturers complex and uneconomical. An overview of most common worldwide codes and standards is given in Appendix M. 

Boilers having attached superheaters must have at least one valve on the superheater. The valves on the drum must be large enough to relieve at least 75% of the total boiler capacity. It is good practice to size the superheater valve to relieve approximately 20% of the total boiler capacity to protect the tubes against overheating. 

Application and sizing of pressure relief valves, pilot-operated pressure relief valves, and safety valves for use on pressure vessels are found in the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Rules for Construction of Pressure Vessels, Paragraphs UG-125 through UG-137. 

The boiler itself should be very efficient to generate steam and for this preparation of oil, air to fuel ratio, stack temperature of flue gases, conversion of furnace oil to LSHS oil, maximum condensate recovery, cleaning of the fire side, replacement of old boilers etc. are the various important factors to be taken care-of. Factors responsible for optimum consumption of steam such as supply of steam at correct pressure, provision of pressure reducing valves, water separators in steam lines for supply of dry steams at requisite pressure, selection of trap of right type and size for efficient recovery of condensate etc. should be considered for the saving of thermal energy. 

Restrictions on the maximum furnace heat liberation allowed vary. The size and number of safety valves may also vary. In addition, personal wishes of the users and their financial and operating policies can influence the design and price of a boiler installation considerably. 

These valves are each sized to pass a steam flow of 945,292 Ib/hr at 1308 psia. This limits steam generator pressure to less than 110% of steam generator design pressure during worst case transients. The secondary safety valves consist of two banks of 10 valves with staggered set pressures. The valves are spring-loaded safety valves procured in accordance with ASME Boiler and Pressure Vessel Code, Section III (see Table 5.2-1). Parameters for the secondary safety valves are given in Table 5.4.13-2. 

Overpressurization of the Reactor Coolant System (RCS) and steam generators is precluded by means of primary safety valves, secondary safety valves and the Reactor Protection System (RPS). Pressure relief capacity for the steam generators and RCS is conservatively sized to satisfy the overpressure requirements of the ASME Boiler and Pressure Vessel Code, Section III. The safety valves in conjunction with the RPS, are designed to provide overpressure protection for a loss-of-load incident with a delayed reactor trip. 

Consider the circulating fluidized bed (CFB) boiler. Figure 1 where typical operating conditions might be 3-5 m/s up to the secondary air injection level and 5-8 m/s above that level at a bed temperature of 1,173 K. Coal with a top size of 6 mm is fed at the bottom of the bed consisting of coal-ash particles of mean diameter 0.3 mm. Entrained particles are collected in a cyclone and recycled to the combustor via a non-mechanical return valve. Part of the heat released from the carbon burning in the bed is extracted by water wall tubes placed in the furnace. The remaining heat leaves the furnace as sensible heat in the flue gas. 

Final control elements In rare instances the final control elements can be duplicated, in cases when the erosive/corrosive or sticking characteristics of the fluid could cause unacceptable downtime or in cases of critical controls (viz, boiler drum level control with control valves in medium-sized power plants). The major cases are as follows ... 

The problems with check valves have been caused by weakness in several activities relative to their performance. Check valve design has been faulty in some instances and has led to failure of internal parts. Application of the valve such as sizing, installation, and location may not be appropriate for system conditions. Because of improper categorization with respect to maintenance, some check valves important to safety have not been subject to routine testing with respect to each of their safety functions. Also, the inservice tests required by NRC regulations through reference to the ASME Boiler and Pressure Vessel Code are not fully sufficient to predict or to indicate valve degradation. Further, inadequate procedures have led to improper valve assembly. 

Are safety valves installed with full-sized discharge piping properly supported and directed to a point of safe discharge and is safety valve set pressure equal to or less than boiler maximum allowable working pressure, because safety valve relieving capacity must be equal to or greater than boiler output ... 

Control of the water level in the steam generator involves much the same problems, due to steam bubbles that are experienced in conventional boilers, with the added complexity that the steam pressure increases as the load on the plant decreases. Sizing of the ports in the feedwater regulating valves must take this into consideration, and the boiler feed pumps must be designed for the no-load, rather than the full-load head requirements. 

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