Manual Blow-off Valves
Boiler blow-off valves are generally used as prescribed by the ASME Power Boiler Code, Section I. The code indicates the basic requirements as well as specific limitations; and generally blow-off valves are designed, selected and applied in accordance with these requirements. The code states:
Each boiler shall have a bottom blow-off pipe fitted with a valve or cock in direct connection with the lowest water space practicable.
- The minimum size of pipe and fittings shall be 1 in. and the maximum size shall be 2 1/2 in. except that for boilers with 100 sq. ft. of heating surface or less, the minimum size of pipe and fittings may be 3/4 in.
- Straight-run globe valves of the ordinary type, or valves of such type that dams or pockets can exist for the collection of sediment, shall not be used on such connections.
- Straightway Y-type globe valves, or angle valves, may be used in vertical pipes, or they may be used in horizontal runs of piping, provided they are so constructed or installed that the lowest edge of the opening through the seat shall be at least 25 per cent of the inside diameter below the centerline of the valve.
- Return connections of the same size or larger than herein specified may be used, and the blow-off may be connected to them. In such case the blow-off must be so located that the connection may be completely drained. All water walls and water screens which do not drain back into the boiler and all integral economizing shall be equipped with drains or blow-off valves conforming to this paragraph (P-308), and paragraphs P-309 and P-311.
From the above it can readily be seem that blow-off valves are somewhat different in design and application than ordinary globe or gate valves. Blow-off valves are usually designed and applied in accordance with these requirements and the requirements of paragraphs 299, 309, 310 and 311, of the code.
Purpose of Blow-off Valves
Since blow-off valves are installed at low points of the boiler water system, it is obvious that one of their principal uses is to help remove suspended solids or impurities, particularly those that tend toward sedimentation. Secondarily then, it is also obvious that is blowing down, a portion of the boiler water is discharged thereby reducing the water level in the boiler. This function therefore aids as a safety measure, in avoiding carryover of water from the main steam drum in the event of high boiler water-level for whatever reason. A third use of these valves, but far less frequent, is the introduction of boiler cleaning acid to the boiler, and again draining of this acid after the boiler is acid-washed. Many boiler operators choose to pump this fluid through the blow-off valves, since they usually are a handy means of ready access to the boiler.
Pressure Ratings Set By Code
The power boiler code also fixes the pressure ratings for blow-off valves. To properly apply blow-off valves one should be familiar with the code requirement for this service.
Paragraph P-310 of the code covers valve pressure ratings. Carbon steel fittings and valves designed to American Standards Association (ASA) requirements for 150, 300, 400, 600, 900, 1500 or 2500 psi are limited by the code under this paragraph, in that they may be used for blow-off line service, provided their adjusted pressure-temperature ratings exceed the maximum allowable working pressure of the boiler by 25 per cent, or 225 psi, whichever is less.
To avoid the need for calculations, the Boiler Code Committee has set up a table listing adjusted pressure ratings for saturated steam-temperature conditions, and the allowable values in blow-off service. For convenience, this is shown in Table I.
Table I:
Maximum allowable boiler pressures for use with American standard steel pipe and flanges, fittings and valves.
Maximum Allowable Pressure
Type of Flange |
|||
Class B |
180 |
150 |
|
Class B |
510 |
415 |
|
Class B |
665 |
540 |
|
Class B |
960 |
785 |
|
Class B |
1360 |
1170 |
|
Class B |
2100 |
1910 |
|
Class B |
3150 |
2975 |
An accurate graphical method for determining the reduced value for blow-off service is shown in Fig. 4. In it the pressure-temperature curve for saturated steam is plotted along with the pressure-temperature curve for a Class "A" steel, 1500 lb. fitting as used in steam service. Another curve is plotted, 225 psi below the valve curve; where this lower curve intersects the steam curve, the blow-off rating is fixed. As graph shows, ASA 1500 psi carbon steel valves have a primary temperature rating of 850°F. They may be used to 2500 psi for saturated-steam service, and to 2325 psi for blow-off service.
To save time, valve selection tables usually list the ASA primary pressure rating, such as 1500 psi, and the ASME code pressure for blow-off, such as 2325 psi. Saturated steam rating, in this case 2500 psi, is not ordinarily listed. Table II (yet to come) is a typical example of condensed data on tandem carbon steel blow-off valves.
Blow-off valves of cast-iron construction are also permitted for boiler blow-off service. Their rating in this service is limited to 80 per cent of the nominal ASA fitting rating. For instance, a 125 psi fitting is limited to 100 psi blow-off service, and a 250 psi fitting is limited to 200 psi blow-off service. Brass or bronze valves may be used, provided they are at least equal to the strength requirements of the American Standard cast-iron fitting which would otherwise be required. Steel blow-off valves are required for boiler pressures over 200 psi. If boiler pressure is over 100 psi and iron blow-off valves are to be employed, they must be equal at least to the requirements of American Standards for 250 psi.
Blow-off vales may be connected as screwed, flanged or welded fittings.
Blow-off Valve Types
There are many different types of blow-off valves manufactured. These could be classified as follows:
Seatless, sliding-plunger type.
Sliding disc, or non-wedging gate type.
Seat and disc, or hard seat type.
Combination of two of the above valves in a common body.
Since the boiler code indicates that on boilers where the pressure exceeds 100 psi, each bottom blow-off pipe should have two slow-opening valves, or one slow-opening and one quick-opening valve, both types are offered by manufacturers. By definition a "slow-opening valve" is one which requires at least five 360-degree turns of the operating mechanism to change from full-closed to full-open, and vice versa. In the above listing it can readily be seen that without gear operation, the sliding-disc type is generally a quick opening valve.
The seatless valve is classified as a slow-opening valve. It has a rotating non-rising stem, which when turned withdraws the plunger so that its ports coincide with the body inlet port. Blow-down flow then is through the lower portion of the plunger and out through the discharge end. This valve is furnished in both the angle, and straightway pattern.
In the sliding-disc type of valve the disc seals the opening through the valve when in the closed position. This disc has a through hole on one side, which is swung in line with the bore of the valve, and through which flow takes place when the valve is open. This is known as a quick-opening valve, and is furnished in a straight-through pattern only. It must always be used in conjunction with a slow-opening valve, for blow-off service.
Since the boiler code permits the use of two blow-off valves in a common body, some manufacturers take advantage of this fact. This arrangement eliminates another bolted, or welded, connection between the valves, and provides a relatively compact unit to be used at each blow-off location. An example of this is the combination shown in Fig. 2. This shows a seat-and-disc type of valve used as the blowing-valve installed next to the boiler, and a secondary seatless valve used as the sealing-valve. Between these two valves there is an opening in the body to allow through-flow.
As previously mentioned, blow-off valves are frequently used for the introduction and drainage of boiler-cleaning acids. This is severe service and manufacturers select materials that can withstand the corrosive effect of this liquid, as well as the erosive cutting action of the blow-down itself. When acid cleaning is anticipated, purchasers of blow-off valves should so indicate. The seatless valve, for instance, would then employ a type 317 stainless-steel plunger, rather than the Standard "Nitralloy"; and the lower gland would be made of "Ni-resist" rather than of chromium-plated steel.
Methods of Operation
Usually, blow-off valves are manually-operated, by handwheel or lever. A handwheel is employed where rotary motion is required to open or close the valve. Normally, clockwise rotation closes the valve. Some manufacturers introduce a gear-reduction for ease of operation. Quick-opening valves are usually direct lever operated, although some use a gear segment or rack arrangement for easier operation.
Remote operation has been introduced in some power plants. Electric motor operators, controlled remotely by push-button stations, have been used in some modern plants where the tendency is toward automation. Others have used hydraulic and pneumatic cylinder operation.
Maintenance of Blow-off Valves
Materials used in modern blow-off valves tend to minimize maintenance. Packing is generally the only important item necessary to be kept in stock. Every time a valve is disassembled new packing should be installed.
Since blow-off valves operate at saturated-steam temperatures, a high-temperature oil is recommended for maintenance lubrication. It is good practice to lubricate a valve just before it is operated, to guarantee clean lubricant in the proper place when it is most needed.
A new development in the blow-off valve field has been the use of hard-seat valves, with integral seats. By depositing the hard-facing material, usually stellite or its equivalent, directly in the body of the valve, the screwed-in-seat connection is eliminated. Formerly valve seats were threaded in, and sometimes sealed by a gasket or seal-weld. Now this is eliminated. Should the integral seat need attention, the manufacturer offers a tool which can recut or regrind the seating surfaces while the valve body remains in place in the pipe line.
Blow-off valves are usually furnished with any one of three kinds of end connections; namely, threaded, flanged or welded. Usually, threaded valves are employed on lower pressure installations. Flanged valves are probably the most popular for general usage, and are used in practically all pressure ranges. However, for the higher pressure ranges (nominally 2500 psi), welding ends are most frequently used. Of welding connections, socket-weld is generally the most popular. Butt-welding ends are provided when required, and may be to standard ASA dimensions.
End connection dimensions are established by the appropriate ASA Standard. These standards cover iron and steel fittings. They cover screwed, flanged, socket-weld and butt-weld types of connections.
Valves are piped, wherever possible, in such a manner as to assure clear drainage to sewer or to blow tank. As was previously stated, the Code places restrictions on the maximum amount of damming allowed in the design of this type of fitting. It is only necessary for the boiler manufacturer to design his piping so as to prevent any trapping action. Blowing hot boiler water, at or near steam temperature, from a boiler into cold residue water (in blow-lines) can cause severe shock to the system.
Blow-off valves in some power plants, due to the design of their boilers, are placed in relatively inaccessible locations. In order to bring the handwheels to positions convenient for manual operation, some valves have been equipped with extension stems; and, in some cases, have had their handwheels supported on floor stands. When gratings or other obstructions come in the vicinity of the valve, this method provides ready accessibility for valve operation. Another method which as been successfully employed is the use of a flexible shaft, to transmit the opening and closing load from the handwheel to the valve. The valve can be provided with an adapter, to mount the flexible shaft, and the handwheel can thus be established where it can best be operated.
Since blow off service dictates the use of a rugged set of valves, utilities (particularly) tend to specify heavy-duty valves manufactured specifically for this purpose.
It is for these reasons that most power engineers and design consultants turn to established manufacturers of this specific equipment, who can contribute their years of experience in valve selections.