Minimum Drum Diameter
Sizing of steam disengaging drums relys on experience as much as specifics, but the following guidelines should be considered.

Except for very special designs, the minimum inside diameter of steam drums for HRSG service should be a minimum of 48 inches.

Steam drum water surge volume should be determined for rate of change due to anticipated process operating conditions and for anticipated rate of change due to system pressure increase or decrease. If not known, a minimum rate of change to be considered should not be less than 20% per minute. The drum sizing must allow for the swell volume to be accomplished without actuating the high liquid level alarm or causing liquid carryover into the saturated steam piping. Likewise, the shrinkage should be accomodated without actuating the low level alarm.

The vapor velocities in the free space above the normal liquid level should not exceed the following velocities at design flow rate and normal operating pressure.

Feedwater Holdup Time
In addition to accomodating the shrinkage volume discussed above, the drum size should allow for a minimum of two minutes holdup time at design flowrate in the advent of a loss of feedwater.

Internals, Separators
The steam drum must be sized to accomodate the internals necessary to meet the guaranteed steam purity requirements, as well as the riser and downcomer conections. The internals include the primary separators, baffles or centrifugal, the secondary separators or dry pipe, as well as the channels or plenums needed for collecting the steam and water mixture from the risers. All steam drum internals should be removable without cutting. All internals should be able to be removed through the drum manhole.

There are many types of drum internals available, so we are only suggesting what may be used. For low pressure drums, under 50 psia, simple primary baffles and secondary dry pipe should be sufficient for most services. For low pressure drums integrally connected to a deaerator, no dry pipe is required. For other pressures and services the use of centrifugal primary separators with chevron secondary scrubbers are recommended. We aslo recommend the use of a dry pie on internal connection of steam outlets even when chevrons are used.

Chevron Scrubbers
The minumum area of chevrons required may be calculated using the following:

So, if you had a single row of chevrons 8" high, you would need 1.5*A_min in length or if you had a double row, i.e., one row on each side of dry pipe, you would need a length of 0.75*A_min. Of course, for larger diameter drums, you could use 12" high chevrons and reduce the length requirement accordingly. You should keep in mind, however, that increasing the height of chevrons reduces the working area in the drum which is needed for drum swell, etc.

Centrifugal Separators
The minumum number of centrifugal separators may be calculated using the following:

We are assuming 12" centrifugals for this case(remember, they must go through our 12"x16" manhole) so it is fairly easy to determine the length required for a single row. But to reduce the required length, you may stagger the cans and/or place them along both sides of the drum. Additionally, the formula is designed to provide a 1 psi pressure loss through the centrifugal separators at design load.

Feedwater Distribution Pipe
The boiler feedwater distribution pipe should have the following minimum pipe inside diameter.

Length of the pipe should be approximately the full length of the drum. Pipe should be secured and supported approximately every 35 diameters. Pipe should be perforated approximately 12 inches on center. Total flow area of perforations should not be less than area of the pipe. Feed pipe should have at least one breakaway joint at connection to external nozzle inside drum. Above 3 inch size should be flanged and below 3" should be threaded. End of pipe should be capped with a vent hole at top. The perforations should be orientated downward and toward the chemical feed pipe. The pipe should be below low liquid level and, if possible, in the stream of effluent coming from the primary separators and going to the downcomers. The entry into the drum must be fitted with a thermal sleeve if required by ASME code. Preferred location is horizonal, on centerline of drum head. If drum has thermal sleeve, it may not be located below horizontal, which would allow debris to collect in sleeve. If the economizer is designed such that steaming can occur at any operating condition, this design is not adequate. Special design for steaming economizer is not covered in this discussion.

Continuous Blowdown Pipe
The boiler continuous blowdown pipe should have the following minimum pipe inside diameter.

To calculate the blowdown flow, you must know the solids in the feedwater. If known, you can base the blowdown flow on the following ABMA standards for drum water conditions, unless the specifications require a more conservative level.

So if you had a feedwater with a total solids of 20 ppm and you were operating at 650 psia, then the blowdown based on percent of feedwater flow would equal 20/2000*100 = 1%, or based on feedwater flow it would equal 20/(2000-20)*100 = 1.01%.

The minimum pipe diameter to be used for this service is 3/4 inch IPS. Length of pipe should be full length of drum. Pipe should be perforated on approximately 12 inch centers. Perforations should be orientated in an upward direction. Blowdown pipe should have a threaded breakaway joint at connection to internal nozzle. The blowdown pipe should be located below the water discharge of the primary separators. If the separators are located on both sides of the drum, the blowdown pipe should be divided and ran on each side.

Intermittent Blowdown Pipe
A rule of thumb to use is for steam flows up to 150,000 lbs/hr, use 1 1/2 inch; and for greater than 150,000 lbs/hr, use 2 inch size. Preferred location is dependent on type of HRSG. In double drum designs, the intermittent blowdown pipe should be located in the mud drum. For single drum designs, the blowdown pipe should be located in the bottom of the drum. The purpose of the intermittent blowdown is to make quick corrections to water level as well as sludge removal. It should be pointed out that there is not much "sludge" in modern boilers with sufficient water treatment. This blowdown pipe may also be used as part of the overall boiler draining procedure.

Chemical Feed Pipe
Minimum internal pipe size should not be less than 1/2 inch IPS. There is no specific sizing rules since normally at time of design, the flow is not known. A rule of thumb to use is for steam flows up to 50,000 lb/hr, use 1/2 inch; for 50,000 to 150,000 lbs/hr, use 3/4 inch; and for greater than 150,000 lbs/hr, use 1 inch size. This pipe should be 304 SS material including the nozzle entering the drum. Good practice is to use a ss sleeved entry nozzle designed such the the chemical feed pipe can be replaced easilly. Pipe should be supported every thirty-five diameters. Chemical feed pipe should be capped and vented at end. Preferred location is where the wash from the feedwater pipe will mix the chemicals well before they can enter the downcomers. Care should be taken so that chemicals cannot be drawn up by the contiuous blowdown.

Other Miscellaneous Nozzles
The minimum connection size for any connection should not be less than 3/4 inch. All nozzle connections up through 2 inch shall be minimum schedule 160.

Manholes
Manholes should be 12 inch by 16 inch or larger and must be equiped with yoke and hinged cover. Sizing must be able to facilitate removal of all internals without cutting.

Steam Outlets
All drums should have a minimum of two steam outlets, manifolded together outside the drum. This reduces the demand on the dry pipe and chevrons, as well as, lowers the horizontal velocity in the vapor space. An exception to this rule may be made for units with a total steam flow of less than 50,000 lbs/hr and there is no superheater. The minimum piping area should not be less than:

Normal Steam Drum Connections

For our sample HRSG that we have frequently referred to in this discussion, our steam drum might look like the one shown below. When we reviewed the natural circulation that would be the case for this HRSG, since it is an O-Frame design, we concluded that we needed 16 centrifugals. Since we have risers entering the drum from both sides, it makes sense to use two rows of centrifugals, so without staggering, we would need at least 8 feet of length. Since we had 28 tubes wide at 4" spacing, the inside width of the evaporator would be 28 * 4/12 = 9.33'. If we add 18" to allow for the casing and structure, then the seam length of the drum would be 10'-8" which is plenty of room for our centrifugals.

Our guide for the chevrons indicates we need a minimum of 12 ft². So if we assume 8" high with two rows, we would need 9 feet of length. With our seam length, we have plenty of room for our chevrons.

Following our rule of two outlet steam connections, the maximum vapor flow at any point in the upper part of the drum would be 104396/4 = 26,099 lb/hr. Using our formula for maximum velocity, V_h, from above, we are okay up to 3.85 ft/s. if we assume the normal water level, NWL,is at centerline of drum, our net flow area is 6.28 ft², and our velocity would be only 1.15 ft/s without discounting the area blocked by chevron hangers, so we are okay.

Now we can check our storage volumes and times from one level to the other using this JavaScript.

The first thing we notice after checking the times is that we don't meet our requirement for 2 minutes between NWL and LLSD. We can achieve this by raising the NWL to 26 inches and lowering the LLSD to 6 inches. But, this shows how tight the design of this drum is due to the short length. We could of course increase the diameter, but this would affect the cost, or we could increase the length, but then we couldn't ship because of the width of the module. Remember, when we change these levels, we must recheck the vapor velocities.