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How do I calculate discharge rate needed for a boiler?


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  #1  
Old 04-23-2009, 11:27:06 AM
MarkP MarkP is offline
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Default How do I calculate discharge rate needed for a boiler?

I have a horizontal firetube boiler that has a heating surface of 125 sq. feet, is rated for 150 lbs, and is app. ( I am guessing) no more than 30 h.p.. How do I calculate my steam generating capacity so I do not get a safety valve with too low of a PPH discharge, or one so high that it sounds like a cannon going off and drops my pressure down "too fast"?

Thanks,

Mark
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Old 04-23-2009, 12:27:36 PM
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WildIrishman52 WildIrishman52 is offline
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Default Re: How do I calculate discharge rate needed for a boiler?

Here is a link that should help you out...

http://www.engineeringtoolbox.com/bo...er-d_1061.html
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Old 04-23-2009, 12:51:31 PM
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Default Re: How do I calculate discharge rate needed for a boiler?

Some "rule of thumb" info on old boilers. 10ft² = about 1 boiler HP. So your boiler will be about 12.5HP. My 16HP Huber has about 162ft² of heating surface. For solid fuel firing size your relief valeve by multiplying heating surface X 7. 125 X 7= 875lb/hr steam. Be sure to specify the set pressure and the amount of blowdown that you want.
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Old 04-23-2009, 02:36:51 PM
MarkP MarkP is offline
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Default Re: How do I calculate discharge rate needed for a boiler?

A thousand thanks folks! Those were a HUGE help!

Mark
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Old 04-23-2009, 09:21:43 PM
DaveWachter DaveWachter is offline
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Default Re: How do I calculate discharge rate needed for a boiler?

you do not want any more than a five lb blow down or you will run out of steam . call steller and give them the poop on your boiler like pressure and heating surface they will calulate your relief valve .also you kneed two ways of puting water into your boiler re. injector or pump have fun but be careful thanks DAVE WECHTER ILL ENGINE NO 135
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Old 04-24-2009, 04:23:42 PM
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Default Re: How do I calculate discharge rate needed for a boiler?

52.65-5 General requirements. (a) Each boiler and its superheater shall be provided with one or more approved safety valves designed, constructed and flow-tested in conformance with Subpart 162.001 of Subchapter Q (Specifications), Code of Federal Regulations of the United States of America, as amended. Boilers of more than 500 square feet of water heating surface but without superheaters shall be fitted with at least two safety valves, and boilers with integral superheaters shall have at least two safety valves attached to the drum and one safety valve fitted to the superheater outlet. On river steam vessels whose boilers are connected in batteries without means of shutting off one boiler from another, each battery of boilers shall be treated as a single boiler and equipped with not less than two safety valves of equal size.

(b) Drum safety valves shall be set to relieve at a pressure not in excess of that allowed by the certificate of inspection. Where for any reason this is lower than the pressure for which the boiler was originally designed, an accumulation test, as provided for in Paragraph (g) of this section, shall be conducted to insure that the relieving capacity of the safety valve is sufficient for the lower pressure.

(c) The setting of the superheater safety valve shall not exceed the design pressure of the machinery plus the pressure drop in the piping from the superheater outlet to the machinery. To prevent damage to the superheater, the drum safety valves shall be set at a pressure not less than that of the superheater safety valve setting plus 5 lbs. minimum plus approximately the normal load pressure drop through the superheater. If there are no intervening valves between the superheater safety valve and boiler, the relieving capacity of the superheater safety valve rated in accordance with the requirements of Subpart 162.001, Subchapter Q (Specifications) Code of Federal Regulations, may be included in determining the total required relieving capacity of the boiler, provided the drum safety valves have at least 75 per cent of the aggregate relieving capacity required. The foregoing requirements do not apply when drum pilot actuated superheater safety valves are used. Installations employing pilot actuated safety valves will be given special consideration by the Commandant.

(d) In the event the maximum steam generating capacity of the boiler is increased by any means, the relieving capacity of the safety valves shall be checked by an inspector and if determined to be necessary, valves of increased relieving capacity shall be installed.

(e) The total rated relieving capacity of drum and superheater safety valves as certified by the valve manufacturer shall not be less than the maximum generating capacity of the boiler as certified by the boiler manufacturer. The safety valves shall be capable of discharging all the steam that can be generated without allowing the pressure to rise more than 6 per cent above the set pressure of the valve.

(f) (1) The boiler manufacturer may certify the normal as well as the maximum generating capacity of the boiler. In the absence of such certification the minimum safety valve relieving capacity shall be determined upon the basis of the pounds of steam generated per hour per square foot of boiler heating surface and the water-wall heating surface, as given in Table 52.65-5 (f) (1) :



(2) In many cases a greater relieving capacity of safety valves must be provided than required by this paragraph, and in every case the requirements of Paragraph (e) of this section shall be met.

(3) The heating surface shall be computed for that side of the boiler surface exposed to the products of combustion exclusive of the superheating surface. In computing the heating surface, only the tubes, fireboxes, shells, tube sheets, and the projected area of the headers need be considered. For vertical firetube boilers, only that portion of the tube surface up to the middle gage cock is to be computed. The minimum number and size of safety valves required shall be determined on the basis of the aggregate steaming capacity and the aggregate rated relieving capacity of the valves shall be in accordance with Subpart 162.001 of Subchapter Q (Specifications), Code of Federal Regulations.

(g) An accumulation test shall be conducted in the presence of an inspector to determine the adequacy of protection provided by the aggregate relieving capacity of the boiler safety valves except in instances where possible damage to superheaters makes this impractical. During this test all steam outlet connections except the safety valves and such others as may be necessary to operate the boiler shall be closed and the fires forced to their maximum capacity for a period of 15 minutes for fire-tube boilers and 7 minutes for water-tube boilers.

(h) On new installations, the safety valve diameter for power boilers, superheaters, or unfired steam generators shall be not less than 1 1/2" nor more than 4". Safety valves of 4 1/2" diameter may be used for replacements on existing boilers or unfired steam generators. The foregoing diameters apply to the valve inlet.

(i) Lever or weighted safety valves now installed may be continued in use and may be repaired, but when renewals are necessary, lever or weighted safety valves shall not be used. All such replacements shall conform to the requirements of subpart 162.001 of Subchapter Q (Specifications), Code of Federal Regulations.

(j) Gags or clamps for holding the safety valve disk on its seat shall be carried on board the vessel at all times.

Courtesy of Modern Marine Engineer's Manual
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Old 04-24-2009, 05:36:28 PM
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Default Re: How do I calculate discharge rate needed for a boiler?

Thanks Gary. While I was looking at the proposed rules in Indiana on the Indiana Historical Steam Assoc web page I notice that you shoul multiply the heating surface time 5. I incorrectly suggested multiplying by 7 in my earlier post.
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Old 04-24-2009, 06:27:33 PM
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Default Re: How do I calculate discharge rate needed for a boiler?

Andy:
You'll have to do some calculating, as you don't want to forget the 8 for Water-wall surfaces!

Also this is Merchant Marine . . . might be slightly different than landlubber's???

Gary K

Last edited by Gary K; 04-24-2009 at 07:48:18 PM. Reason: Add some . . . subtract some!
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Old 04-25-2009, 11:09:50 AM
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Default Re: How do I calculate discharge rate needed for a boiler?

Andy:
I Googled in Steam Safety Valve Relieving Capacity, which lead me to this website, and I seen their chart was similar to the one I posted. Click on the website link below and check it out:

http://www.lrc.ky.gov/kar/815/015/051.htm

Apparently 7 is a rule of thumb being used, as I've read that before? Unless I'm understanding this chart I posted the wrong way . . . the heating surface of the tubes is calculated by 5, and the firebox (water-wall surface) by 8 on a hand-fired boiler.

Example: A 1909 20 h.p. CASE steam traction engine has 225.2 sq. ft. of heating surface.
The firebox is 42" long, 27" wide, and 35 3/4" high. It has 47 - 2" diameter tubes 90" long.

Then;
2 x 3.1416 x 90 = 565.488 sq. in. (per tube).
565.488 x 47 = 26577.936 sq. in. (total tubes).
26577.936 / 144 = 184.569 sq. ft. heating surface (tubes).
225.2 - 184.569 = 40.631 sq. ft. heating surface (firebox).

5 x 184.569 = 922.845
~8 x 40.631 = 325.048
Total ~ ~ ~ ~ 1247.893 lbs./hr.

The other way . . . which is simplier; 7 x 225.2 = 1576.4 lbs./hr.
A difference of; 1576.4 - 1247.893 = 328.507 lbs./hr.

Bear in mind the chart states . . . Minimum!


Gary K
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Old 04-27-2009, 08:12:25 AM
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Default Re: How do I calculate discharge rate needed for a boiler?

I think that using 5 or 7 will give safe performance. Many of us think that bigger is better including relief valves. I think that you can have a pop valve that is to big for your boiler. An oversized relief valve will cause an abrupt drop in pressure and can cause priming or worse just like when you open a throttle to fast or encounter a sudden heavy load.
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Old 04-27-2009, 03:18:17 PM
Richard Miller Richard Miller is offline
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Default Re: How do I calculate discharge rate needed for a boiler?

We had a hand fired vertical fire tube boiler with 36 sq. ft. of heating surface with 1148 lbs/hr on a 3/4 safety valve. While discharging steam the boiler itself was priming and shaking on it's base. Inspector did his calculations and came up with 643 lbs/hr and this safty valve works fine. So yes you can have to much capacity.
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