Blowdown vessels Meeting the requirements of HSE PM60

Transcription

1 Blowdown vessels Meeting the requirements of HSE PM60

2 BDV60 blowdown vessels A complete range of equipment for the safe disposal of boiler blowdown Boiler blowdown Steam boilers must be blown down to remove contaminants, as too high a level will cause foaming, and scaling of the boiler tubes. A build-up of solids at the bottom of the boiler could also have serious consequences. Most modern boilers are fitted with TDS controls to remove dissolved solids, and a blowdown valve that is opened at regular intervals to remove precipitated solids. Because the bottom blowdown valve is opened once per day for just a few seconds, it is not economical to recover the heat, yet water cannot be discharged to drain unless it is cool. Nowadays, blowdown vessels have replaced blowdown pits as the recommended way of cooling this water. Blowdown from TDS controls should ideally be connected to a heat recovery system, but where this is not practical, TDS blowdown may also be piped to a blowdown vessel. Regulations The key UK regulations are those issued by the Health and Safety Executive and British Standards. The Health and Safety Executive Guidance Note PM60 states that blowdown vessels should normally be provided for all new installations instead of blowdown pits. The blowdown vessels should be constructed to PD 5500 and should be regularly inspected by a competent person such as an insurance inspector. Lifting lugs for safe positioning of vessel on site. Internal stainless steel wear plate. Level control chamber and gauge glass blowdown. Discharge from automatic TDS control system. The implications of PD 5500 construction include design approval, material certification, independent approval of the welders and welding processes. On completion, a data dossier is prepared and issued to the customer. Spirax Sarco blowdown vessels fully comply with all these requirements plus the requirements of the European Pressure Equipment Directive 97/ 23/ EC. Blowdown from the bottom of the boiler. User benefits l Three inlets as standard with dedicated bottom blowdown connection. l Fully complies with the requirements of the European Pressure Equipment Directive 97/ 23/ EC. Line drain valves for emptying a boiler that has been shut down. l Full compliance with PM60 and PD l Documentation in compliance with PD 5500, Modules B and F. l Complete systems normally readily available from stock. l Quick and easy to install and maintain. l Safer and cheaper than a blowdown pit. l Spirax Sarco's guarantee of worldwide knowledge, service and technical support. 2

3 The BDV60 blowdown vessel has been designed with three inlets as standard. Dry vented flash steam One inlet is dedicated to bottom blowdown, whilst the other two inlets are interchangeable between TDS blowdown and level gauge glass blowdown. A typical two-boiler application is shown below. Separated water to drain Inlet from blowdown vessel Vent head with internal separator removes moisture from the steam. Pressure gauge for monitoring operation. Outlet to drain maintains internal standing water level to cool incoming blowdown. Oval inspection opening with non-asbestos gasket. Cooling water system for application where frequent blowdown is necessary. Fully complies with the requirements of the European Pressure Equipment Directive 97/23/EC. Drain valve for removing sludge from the blowdown vessel. The blowdown vessel is designed and constructed to PD 5500 complete with full documentation. 3

4 Sizing and selection The selection of a blowdown vessel depends on the flowrate and the proportion of flash steam released. The following factors affect the selection:- n The boiler pressure. n The number of boilers. n The duration of blowdown. n The blowdown line size. n The length of the blowdown line between the boiler and the blowdown vessel. n The blowdown regime. For example, the sizing below assumes the blowdown line has an 'equivalent straight length' of at least 7 m. When estimating 'equivalent straight length' the lengths shown in Table 1 should be added to the length of straight pipe to allow for valves and fittings. From Table 1 it will be seen that in most practical applications it would be unusual for the equivalent straight length to be less than 7 m. If the length is less than 7 m multiply the actual boiler pressure by 1.15 before using the sizing table, Table 2. The blowdown regime: n Typical normal bottom blowdown of, perhaps, 5 seconds duration. n Weekly low level alarm testing where practice is in accordance with PM5 - steam down from normal level to first low water level and blow down from first to second low water level. n Blowdown discharges from sequencing valves on external chambers, gauge glasses etc. n Automatic TDS control discharges. The selection table (Table 2) only applies if the blowdown duration is no longer than 20 seconds in total, starting with a cold vessel (water at a temperature of 15 C to 20 C). If this time is exceeded, carryover of water with the flash steam may occur through the vent. The water may also be too hot to discharge safely and legally into a public drainage system. Always incorporate a cooling water system for multi-boiler applications or in instances where the frequency or duration of blowdown is considerably in excess of PM60 or PM5 requirements. If there is any doubt about correct vessel selection, please contact Spirax Sarco outlining the specific conditions and blowdown regime. How to select the blowdown vessel: Step 1 Step 2 Step 3 Step 4 With the aid of Table 1, determine the equivalent straight length of the blowdown line. Use Table 2 to initially establish the correct size of the vessel. Note: if the result of Step 1 is less than 7 m, multiply the boiler pressure by If the vessel is only used in the conditions set out above, then continue to Step 4. Using Table 4, page 5, containing the vessel data, establish the volume of standing water in the selected blowdown vessel. This volume should be at least twice that of the maximum volume of blowdown. The maximum volume of blowdown is usually the volume discharged when blowing down from 1st low level to 2nd low level alarm. If this volume is unknown, it can be calculated with the aid of Graph 1, where details on its use are given. If it is determined that the volume of standing water is insufficient, then a larger blowdown vessel must be selected in order to satisfy this requirement. With the aid of Table 3, the correct vent head can now be selected to suit the vessel. The selection is now complete. Selection example: For a 10 bar g boiler with 40 mm blowdown line at least 7 m long select, from Table 2, a BDV60/5. From Table 3 select a VH6 vent head. Blowdown flowrate (I/s) Table 1 - Equivalent straight lengths Blowdown line 25 mm 32 mm 40 mm 50 mm size (1") (1¼") (1½") (2") Pipe fitting or valve Equivalent length in metres Long radius bend Manifold inlet Globe valve Check valve Blowdown valve Table 2 - Blowdown vessel selection Blowdown line 25 mm 32 mm 40 mm 50 mm size (1") (1¼") (1½") (2") Boiler pressure Blowdown vessel bar g psi g BDV60/_ Note: For intermediate pressures go to the next higher pressure. Graph 1 - Blowdown flowrates This graph will give the flowrate of blowdown in litres per second. When this value has been read, multiply it by the duration of blowdown (seconds), and the answer will be the maximum volume discharged. Blowdown line size 50 mm mm Boiler pressure (bar g) Table 3 - Vent head selection 32 mm 25 mm 20 mm Note: The vent head required depends on the vessel selected For a BDV60/3 select a VH4 vent head For a BDV60/4 select a VH4 vent head For a BDV60/5 select a VH6 vent head For a BDV60/6 select a VH6 vent head For a BDV60/8 select a VH8 vent head For a BDV60/10 select a VH8 vent head For a BDV60/12 select a VH10 vent head 4

5 BDV60 technical data Table 4 - Sizes, pipe connections, dimensions, weights and capacities (approximate) in mm, kg and L Blowdown vessel type BDV60/3 BDV60/4 BDV60/5 BDV60/6 BDV60/8 BDV60/10 BDV60/12 A Flanged PN16* B Flanged PN16* C Oval Height Width D Sizes, pipe E connections F and G dimensions H J K X Y Number of legs Weights Empty Full (e.g. for hydraulic test) Capacities - standing water * Note: Flanged BS 1560 ANSI Class 150 or ANSI Class 300 connections are available at extra cost. For further details contact Spirax Sarco. K K Pressure/temperature limits D A vent Please note: For the application of boiler blowdown and to comply with the HSE Guidance Note PM60 the vessel internal pressure should not G exceed 0.35 bar g. F Body design conditions PN16 PMA Max allowable pressure 7 bar 171 C 2" BSP pressure TMA Maximum allowable temperature bar g gauge Minimum allowable temperature 0 C PMO Maximum operating pressure re. PM bar g H 2" BSP drain Inlet 3 DN25 PN16 (Level control chamber and gauge Inlet 2 glass blowdown or TDS blowdown) DN25 PN16 (TDS control system or level control chamber/gauge glass blowdown) Inlet 1 DN50 PN16 (Main bottom blowdown) The BDV60 is constructed from carbon steel to meet all the requirements of PD The paint finish is silver. E Name-plate * B outlet Y ¾" BSP cooling water inlet 1" BSP temperature sensor J X * TMO Maximum operating temperature re. PM C Minimum operating temperature 0 C Designed for a maximum cold hydraulic test pressure of 12 bar g PTMX Maximum test pressure 17 bar g Safety information, installation and maintenance For full details see the Installation and Maintenance Instructions supplied with the product. Note: There is a second inspection opening on the BDV60/6, BDV60/8 and BDV60/10 which is fitted 180 to the top opening. Other sizes have a single central opening, similar to the one shown on page 3. Maintenance note: The vessel must be drained every six months to remove concentrated blowdown water/sludge. Before reusing, the vessel must be refilled with fresh water. Blowdown vessels should be thoroughly examined by a 'competent person' every 14 months or at every major boiler inspection. Spare parts: New gaskets for the inspection openings are available as spares from Spirax Sarco. How to order Example: 1 off Spirax Sarco BDV60/5 blowdown vessel and a VH6 vent head to suit a 10 bar g boiler having a 40 mm blowdown line. 5

6 The BDV60 system VH vent head Stainless steel vent head which fits on top of the flash steam vent pipe. The vent head incorporates an internal separator to remove water spray from the steam and discharge it separately through the drain connection. Dimensions (approximate) in mm Vent head type VH4 VH6 VH8 VH10 Inlet size Drain size 1" BSP 1" BSP 1½" BSP 1½" BSP H W For full details see separate literature. H Drain connection W Inlet connection with plated carbon steel lap joint flange to suit PN16 MF manifold For blowdown vessel inlets on multi-boiler installations. Available with manifold size DN50 flanged PN16 or DN25 flanged PN16 and with 2 (MF2) to 6 (MF6) branches sizes 15, 20, 25, 40 or 50 mm. Note: 40 and 50 mm are only available in DN50 size. Carbon steel welded construction with swept bends. Hydraulically tested to 24 bar g. Paint finish is grey DCV2 check valve Disc check valve which fits between flanges. Stainless steel construction. For further details see separate literature. G3 stop valve and line drain valve Cast steel stop valve with stainless steel valve seat and disc. Flanged PN40 but suitable for use with PN16 flanges. Dimensions (approximate) in mm Valve size DN15 DN20 DN25 DN32 DN40 DN50 A B A B 6

7 Pressure gauge A C B Useful to ensure that the vessel is not pressurised during blowdown. Improves safety since any pressure would give early indication of pipework blockage. The pressure gauge set consists of:- A 4" dial pressure gauge bar (0-23 psi) B 2" BSP 'U' syphon C 2" BSP gauge cock Steel Brass Cooling water system Designed to cool the standing water in the blowdown vessel prior to the next blowdown discharge into the vessel. The system is thermostatically controlled and injects cooling water into the blowdown vessel when standing water exceeds a set temperature (typically 40 C). This equipment is particularly useful when several boilers have to be blown down in succession into the blowdown vessel. The interval between blowdowns needs to be several minutes in order for the cooling system to perform its function effectively. A B C D A E F The system consists of:- A¾" BSP non-venting ball valve B¾" BSP LCV1 check valve C¾" BSP SBRA control valve D¾" BSP Fig 12 Y-type strainer E Sensor pocket F Temperature controller For full details see separate literature. Nickel plated brass Bronze Bronze Brass Brass 128 range 20 C to 110 C Caution: Where the cooling water is likely to give rise to scale formation, the advice of a water treatment specialist should be sought. Vessel drain valve 2" BSP nickel plated brass non-venting ball valve on the bottom connection of the blowdown vessel. Regular opening ensures that the vessel is kept clear of sludge. 7

8 How to specify A typical specification for the blowdown vessel shown on pages 2 and 3 for a two boiler installation complete with all the ancillaries recommended in PM60 might be as follows:- Blowdown vessel 1 - Spirax Sarco blowdown vessel type BDV60/5 designed, constructed and tested to PD Stainless steel vent head type VH6 For main blowdown from bottom of boiler: 1 - Inlet manifold: type MF2 (DN50), branch sizes DN40 PN Check valves type DCV2, DN Stop valves type G3, DN40 PN16/ Line drain valve type G3, DN40 PN16/40 For auto TDS control discharges: 1 - Inlet manifold: type MF2 (DN25), branch sizes DN25 PN Check valves type DCV2, DN Stop valves type G3, DN25 PN16/40 For level controls blowdown: 1 - Inlet manifold: type MF2 (DN25), branch sizes DN15 PN Check valves type DCV2, DN Stop valves type G3, DN15 PN16/40 Other fittings recommended in PM60: 1 - Blowdown vessel drain valve, 2" BSP 1 - Blowdown vessel pressure gauge set 1 - Cooling water system Some of the products may not be available in certain markets. Spirax-Sarco Limited, Charlton House, Cheltenham, Gloucestershire, GL53 8ER UK. Tel: +44 (0) Fax: +44 (0) Enquiries@SpiraxSarco.com Internet: Copyright 2005 Spirax Sarco is a registered trademark of Spirax-Sarco Limited 8 SB-P AB Issue 7