[Steam-engine] Lap Seam Boiler Pressure

Lawrence Swanz LSwanz at superioriron.com
Thu Jul 7 04:49:14 PDT 2005


Chas,

I recieved a message back from the mailing group that the message I sent eariler was too large and it bounced back to me.  So I am going to copy/paste a portion of the information below.  I am going to cut some parts out of the message, as it pertains to Minnesota.  However, if you wish to have the complete document you may email me off list and I will send it to you.  Lots of luck.

Lawrence J. Swanz
Superior Iron Co.
763/428-2211  ext. #223
lswanz at superioriron.com





Hobby Boiler Inspections

	Under State of Minnesota Statute 183.411 all Hobby Boilers shall be inspected every 2 years. The inspections shall alternate between an ultrasonic examination to determine condition and thickness of the boiler pressure retaining boundaries and for computing the MAWP, and a hydrostatic pressure test to determine the integrity of the pressure retaining boundaries.
	Jacketed boilers shall have the jacket removed prior to inspection.
	For the ultrasonic examination the boiler shall be opened for internal inspection. All hand holes and inspection openings shall be opened for access to the watersides. All surfaces shall be as clean as possible, with any loose scale or sediment removed from inside the boiler. The boiler tubes, smoke box and firebox shall be as clean as possible, with the grates and the ash pan removed whenever possible. The fusible plug shall be removed and cleaned for inspection. At the Boiler Inspector's discretion, firebox doorframes and hitches or other attachments may need removal to determine if any deterioration has occurred under them.
	The entire hobby boiler, including the safety valves, injectors, gage glass, piping, valves, fittings and engine shall be visually examined to determine the hobby boilers condition. 
	All internal surfaces, including stays, shall be thoroughly inspected for any damage, deterioration or breakage. Any accumulations of scale, pitting or breakage shall be noted.
	The boiler tube ends shall be properly rolled, with no signs of leakage. The tube ends must be properly beaded over. The boiler tube sheets shall be ultrasonically tested to determine if any thinning has occurred, paying close attention to such areas as the bottom of the front tube sheet, near the front hand hole opening, where soot may have eroded the tube sheet or eroded the rivets on the tube sheet to shell connection, or inside the boiler where scale may have accumulated and eroded the boiler metal. 
	
	A firm that holds a National Board of Boiler & Pressure Vessel Inspectors "R" stamp and is familiar with hobby boiler repairs shall make all welded repairs.  The National Board Inspection Code shall always be consulted, prior to recommending any repairs. Personnel that are familiar with the type of repairs that are needed shall complete repairs such as re-tubing or re-piping of the boiler. 
	The Boiler Inspector shall require Material Test Reports or Certificates of Compliance for materials such as boiler tubes and boilerplate. Piping, valves and fittings shall be properly identified and rated for the pressure and temperature service conditions that are required for the installation.

	Note: ultrasonic thickness measurements should be taken on all boiler surfaces where the inspector has concerns about the boiler pressure containing capabilities. Whereas we currently have the capability of recording only 12 thickness readings on our computer system, that does NOT imply that only 12 readings shall be taken!
	Ultrasonic thickness readings are taken at various locations on the bottom of the boiler barrel, and inside the firebox on both the boiler bottom right and left sides and on the crown sheet. Special care should be taken around boiler attachments, where scale may have accumulated inside the boiler and on the crown sheet around the fusible plug. The lowest 3 readings for all 4 locations are recorded (12 recorded readings total) and readings are also taken and recorded at the barrel riveted joint to calculate lap joint efficiency. 

	The lowest thickness reading on the boiler barrel is used in the following formula:

	P=T x TS x EFF
	         R x FS	

	This will give the MAWP based on boiler barrel thickness.
	Where:
	P = maximum allowable working pressure, psi (MAWP)
	T = minimum thickness in plate, in.
	TS = tensile strength, psi (normally 55,000 unless stamped on boiler shell)
	Eff = joint efficiency (average or calculated)
	R = radius of boiler barrel, in.
	FS = factor of safety (5 for ASME Code boilers, 6 for non-code boilers)
	
	Note- average joint efficiency's are used on butt-strap joints as follows:
	Double riveted butt strap = .82
	Triple riveted butt strap = .87
           	Quadruple riveted butt strap = .94
	The lap-seam joint efficiency's are calculated using the following formula:

	E= (P-D)
	        P 
	Where:
	E = joint efficiency
	P = pitch of rivets in outer row, in.
	D= diameter of rivet after driving, in.




	
 	The diameter of rivet after driving is based on plate thickness at lap joint (Note: 
	Use UT test reading to determine plate thickness at lap joint)
	Thickness of plate          Diameter of rivet after driving
	.25				.6875
	.28125				.6825
	.3125				.75
	.34375				.75
	.375				.8125
	.40625				.8125
	.4375				.9375	
	.46875				.9375
	.50				.9375
	.5625				1.0625			
	.625				1.0625
   
	The lowest thickness reading of the 9 readings taken inside the firebox-stayed 	areas is used in the following formula:

	P= t x t x S x C
	          p x p

	This will give the MAWP based on boiler-stayed area.


	Where:
	t = minimum thickness of plate, in.
	P = maximum allowable working pressure, psi
	S = maximum allowable stress given in table PG-23.1, psi  
	       (Note "S" value = 13,800 psi for SA-285 GR-C @ 700 F)
	p = maximum pitch measured between straight lines passing through the centers of the stay bolts in the different rows, which lines may be horizontal and vertical, or radial and circumferential, in.
 	C = 2.1 for welded stays or stays screwed through plates not over 7/16 in. in thickness with ends riveted over.
	C = 2.2 for welded stays or stays screwed through plates over 7/16 in. in thicknes with ends riveted over.
	C = 2.5 for stays screwed through plates and fitted with single nuts outside of plate, or with inside and outside nuts, omitting washers.
	C = 2.8 for stays with heads not less than 1.3 times the diameter of the stays screwed through plates or made a taper fit and having the heads formed on the stay before installing them,
		and not riveted over, said heads being made to have a true bearing on the plate.
	C =3.2 for stays fitted with inside and outside nuts and outside washers where the diameter of the washers is not less than 0.4p and thickness not less than t. 
	
	After the MAWP is calculated for the boiler barrel and stayed areas the results are compared and the lowest pressure of the two becomes the MAWP of the boiler. Common sense is encouraged in this area! (Note: the boiler MAWP shall never exceed the original design pressure of the boiler or the original design pressure of the engine.) 

The ASME Code Safety valve for the boiler must be set at or below the MAWP of the boiler with a relieving capacity that shall be calculated for the boiler by multiplying the square feet of boiler heating surface x 5 for hand fired fire tube boilers. Only the heating surface above the grates need be considered. The following chart may determine heating surface of the tubes:
	Outside tube diameter		Heating surface per ft.		Heating surface per in
	2"				.5236 sq. ft.			.0436333 sq. ft.
	2.5"				.6545 sq. ft.			.0545416 sq. ft.
	3"				.7854 sq. ft.			.06545 sq. ft.
	3.5" 				.9163 sq. ft.			.0763583 sq. ft.
	4"				1.0472 sq. ft.			.0872666 sq. ft.





More information about the AT mailing list