DESUPERHEATING CONTROL VALVE STRAIGHT-WAY SERIES

Transcription

1 1 DESUPERHEATING CONTROL VALVE STRAIGHT-WAY SERIES

2 DESUPERHEATING CONTROL VALVE STRAIGHT-WAY SERIES Description valve, derived from series, is provided with straight-way globe body and cage trim. The pressure reduction is performed through two stages; the first one is intended for flow control, while the second one is a fixed throttling section. The valve may have different performances and applications, also depending on flow orientation, either over or under the plug. The 1 st stage is provided with single or multiple cage trim, same as series. The 2 nd stage may consist of a silencer installed on valve outlet, or, for less severe applications, of a drilled basket mounted under the seat ring; such a basket also protects the body from the erosion when flow direction is over the seat model is provided both with the basket under the seat ring and the silencer at body outlet. The aforementioned silencer may be either single or multiple cage type (max 3), or consist of an integral Limiphon disk stack welded to the outlet connection. For sizes over 12" the HAS absorption silencer (reactive type) is also available. Only multistage models, mainly the ones provided with Limiphon stack, may be considered as silencers. As a matter of fact, in aforementioned cases, part of the noise generated by the pressure reduction in the first stage is absorbed and transformed into heat inside the silencer, independently from the flow rate. The steam coming out from the silencer enters the desuperheater injection chamber through an annular section where the velocity is kept at a suitable value to enhance the desuperheating efficiency. Performance of desuperheating system depends on the type of adopted devices and are described in relevant technical pamphlets, where piping characteristics (size, velocities, straight lengths, etc.) and minimum probe distance for temperature control are shown. 2

3 Applications low/medium capacity pressure reduction/desuperheating processes - low/medium pressure turbine by-pass with operating temperature lower than 500 C cogeneration plants, even for continuous control services requiring tight shut-off in closed position good performances under the acoustic view point, especially with limited turn-down some limitations as far as p 1 /p 2 ratio is concerned, mainly in models provided with cage silencer Codification X X Desuperheater type undefined LFP fixed area nozzle LVL, LVP or LVC spring type nozzle LVM multiple Spraysat steam-assisted fixed nozzle Downstream silencer type undefined without silencer (same as with outlet desuperheater) disk or drilled basket type silencer (max 3 stages) HAS reactive adsorption silencer (max 3+1 stages) Limiphon silencer (5 to about 20 stages) Basket under the seat without silencer - flow over the seat Basket under the seat + silencer - flow over the seat The basket under the seat ring is sometime convenient when a low pressure drop silencer (e.g. single cage type) is provided; it is normally useless where multi-cage, HAS or Limiphon silencers are installed. WATER STEAM series valve - flow-to- close - piloted plug - Limiphon outlet silencer - injection chamber provided with LVL spring nozzle type 3

4 Technical features body - straight-way globe same as series - cast construction with bolted bonnet - available sizes: 6", 8", 10", 12", 14", 16", 18", and 20" - ratings: up to ANSI 1500 from 6" to 12", up to ANSI 600 from 14" to 20" trim - 1 st stage consisting of single (std. GBR or GBRS) or double drilled cage - linear intrinsic characteristic - quick-change seat ring and plug are same as series - a removable drilled basket is fitted under the seat ring of and 90 models; in model such a basket acts as 2 nd reduction stage external silencer - consists of a single or multi stage (max 3) drilled basket in and 90 models and of a solid Limiphon disk stack in model - external silencer is always welded to outlet body connection; big silencers may be supported by baffles - valves bigger than DN 10" may be equipped with HAS silencer ( construction materials adsorption silencer) - body and bonnet normally in C and Cr-Mo steel according to steam pressure and temperature - trim components in stellited and nitrided F6NM steel and 17-4PH for temperature up to 350 C - silencer normally in Cr-Mo steel - Limiphon stack in X19CrMoVNbN11.1 steel and in AISI 430 for temperature up to 400 C leakage class (according to IEC ) - unbalanced plugs: V class or better - teflon or piloted balanced plugs: V class - balanced plugs provided with graphite inserts: IV S1 class V class only with special pneumatic, electric or hydraulic actuators STEAM WATER flow-to-close - quick-change design - removable seat ring and protecting basket - piloted plug - tight shut-off - single cage silencer - injection chamber provided with liner and LVM desuperheater Typical application: LP by-pass - low p 1 /p 2 ratios - noise level up to 95 db(a) for bare pipe 4

5 Selection flow rate on low pressure services and not severe allowable pressure level - no turn-down limit ideal solution for low pressure medium capacity turbine by-pass - combined cage arrangement of 1 st and 2 nd stage enables to perfectly meet such application requirements as far as process control and generated noise are concerned low noise on low/medium pressure and flow rate services - limited turndown some capacity and turn-down limits - suitable solution for low noise and p and p 1 /p 2 values applications mounting a basket just under the seat ring makes the design very compact and economic, suitable for limited p 1 /p 2 ratios (max 1:6 approximately) and only for flow over the plug - desuperheating efficiency of this model is basically worse then in other models; Spraysat injector (1-5986) is often preferable alternate design for er p 1 /p 2 ratios or lower allowable noise. Aforementioned selection criteria are summarized in the table shown at page 5. flow-to-open / flow-to-close - flow-to-close direction (over the plug) is selected when a tight shut-off is required for temperature services - in such a case piloted plugs may be used also with conventional pneumatic actuators. Balanced plugs provided with special inserts available only with hydraulic, electric or special pneumatic actuators - the flow-to-open design has no particular limitation only when it is possible to use unbalanced plugs (low p s and seat diameters) or the temperature allows the use of Teflon seal rings. WATER STEAM flow-to-close - flow control by single cage - drilled basket under the seat - unbalanced plug - temperature service and tight shut-off - actuator must be hydraulic or electric type - injection chamber provided with Spraysat 5

6 STEAM CONFIGURATION p P1/P2 NOISE TURNDOWN FLOW RATE CONTROL 10 low low very excellent poor 40 satisfactory 50 satisfactory very good 60 very very low excellent 80 low poor 90 satisfactory good 6

7 Sizing Models provided either with internal or external cage silencer may be sized with good accuracy by IEC equations and by selecting DN and port according to the flow coefficient table. Models provided with downstream Limiphon stack must be instead sized by Parcol proprietary criteria, also because in this case silencers are defined according to specific application requirements. Also for noise pressure level prediction of series valve, Parcol follows general principles of IEC standard, but uses a proven proprietary method which takes into account the special configuration of valve+desuperheater assembly. For instance the acoustic benefit produced by downstream injected water is taken into account by a correction factor which is a function both of injector type and the ratio w between water and steam flow rate. The acoustic benefit is maximum where injection is performed by LVM multiple nozzles; it can be calculated by the equation: L w = 38. w 1,5. 7

8 FLOW COEFFICIENTS Cv (1) DN port seat dia. travel /50/60 1 st stage (2) (total) " " " " (1) Linear characteristic - standard drilled cage. (2) General Cv of valve provided with external cage silencer is normally 70% of the one listed for 1 st stage because Cv of 2 nd stage is normally equal to the one of 1 st stage. Different sizing is anyway possible for special applications. PARCOL SpA - Via Isonzo, Canegrate (MI) - ITALY C.C.I.A.A Fiscal code & VAT no. (IT) Telephone: Fax: sales@parcol.com - Issue ACA