Patent Application
20160355195
The invention relates to a manway cover and nozzle for a railway tank car. Specifically, the invention relates to improvements in the manway cover and nozzle system which prevent over-compression of the manway cover gasket and deformation in the manway cover and/or nozzle.
The conventional railway tank car comprises an opening or “manway” on the top for loading, venting or maintenance purposes. The manway includes a sidewall or “nozzle” defining the opening, and a cover received over the nozzle and bolted on. The bolts (sometimes called “eye bolts”) are generally attached to the nozzle with respective eye-bolt lugs which attach an end of each respective bolt to the side of the nozzle in a pivoting arrangement. An end of the bolt opposite the lug is received in a slot formed in the periphery of the cover defined between a pair of “ears.” A nut and washer bear on the top surface of ears to close the cover and compress the gasket. Association of American Railroads (“AAR”) Standard M-1002, which is incorporated by reference, governs manway cover specifications. (Reference to any published standard refers to the standard in effect on the filing date of this application.)
It has been found that excessive torqueing of manway cover bolts may result in gasket deformation, sometimes referred to as “cold flow,” resulting in seal failure. Repeated excessive deformation, may reduce gasket life cycle. Over time, over-tightening may cause deformation of the manway cover itself.
Ideally, a manway cover is tightened in stages, with the bolts being tightened manually in a star pattern until a specified assembly torque is reached. Assembly torque may vary, depending on the number of bolts and the gasket material, but a typical specified assembly torque for a six bolt manway cover falls in the range of 80 ft·lb to 120 ft·lb, with a specified maximum of around 200 ft·lb. In practice, however, much greater torques are applied to the bolts, upwards of 400 ft·lb. This is because, in the field, the manway cover bolts may be machine-tightened via impact wrench in a single pass, following a circle pattern, for example. In the case of over-tightening, the pattern of stress on the gasket may be localized around the eye-bolt positions, which leads to gasket failure. In an extreme case, the ears of the manway cover become deformed.
One object of the invention is to limit gasket overcompression to reduce or eliminate seal failure in a tank car cover.
Another object is to prevent gasket deformation beyond specified limits to improve the life cycle of the gasket.
Still another object of the invention is to prevent cover deformation in the process of tightening manway cover bolts, and more specifically to ensure that the most likely point of deformation is at the eye bolt itself, rather than at the manway cover or nozzle.
Still another object of the invention is to ensure that stress levels in the cover system are not localized around the eyebolts and remain outside the plastic deformation regime at specified maximum loading, thereby reducing the likelihood of failure of a manway cover seal and improving life cycle times of the equipment parts.
These and other objects of the invention are achieved, according to one aspect of the invention, with a manway cover system for a railway tank car comprising: a nozzle having a top edge defining an opening in the tank car, a manway cover received on the nozzle, the cover having a plurality of slots in a peripheral edge thereof for receiving bolts to tighten the manway cover on the nozzle, each of said plurality of slots defined by two ears defining opposite sides of the respective slot. A plurality of bolt lugs is attached on a vertical side surface of the nozzle securing a plurality of respective bolts in a pivoting relationship. A gasket is positioned between the edge of the nozzle and the manway cover. A top surface of each of the plurality of bolt lugs is machined, and a predetermined distance is maintained between the machined top surface of each of the plurality of bolt lugs and the top edge of the nozzle.
At least one projection or “stop” extends downwardly from the peripheral edge of the cover and has a horizontal bottom surface adapted to contact the machined top surface of the bolt lug when the cover is closed on the nozzle. The distance between the horizontal bottom surface of the stop and the machined top surface of the lug may provide for a specified amount of compression of the gasket, such as 25% to 60% of the uncompressed thickness of the gasket, before the stop contacts the top surface of the lug.
In another aspect the invention is a manway cover used in the system, the cover having a projection extending downwardly from each of the ears on the peripheral edge of the cover, and each projection having a machined bottom surface. The machined bottom surfaces of the projections abut the machined top surfaces of respective bolt lugs at a predetermined amount of compression of the gasket.
In still another aspect, the invention is a method of limiting tensile and bending stresses in a manway cover system, comprising machining the top surface of each of said plurality of bolt lugs; machining the bottom surface of each of said projections on the manway cover; and maintaining a predetermined distance between the machined top surface of each of said plurality of bolt lugs and the top edge of the nozzle. Maintaining this predetermined distance within certain tolerances ensures that tightening the bolts to a desired assembly torque causes the machined bottom surface of the projection to abut the machined top surfaces of the bolt lug when the gasket is compressed in a range of 25% to 60%, of its uncompressed thickness. This amount of compression applies to elastomeric gasket materials. In some instances, a gasket material may be thinner, and it may compress without incident to a greater or lesser extent. The design dimension of the compression stop may be adapted in accordance with the thickness of the gasket material and the acceptable amount of deformation, which information is generally readily available from the manufacturer. With the compression stops according to the invention the distribution of stresses on the gasket is more even and is not localized at the eyebolts. Thus, even at specified maximum loading of the eye bolts on the cover, the pressure developed on the gasket at the eye bolt locations is not more than 10% greater than the pressure developed at any other point on the top edge of the nozzle.
The Figures are schematic and not drawn to scale. Some features, not necessary for an understanding of the invention, may be omitted in certain views to better illustrate other features.
Directions and orientations herein refer to the normal orientation of a railway car in use. Thus, unless the context clearly requires otherwise, the “longitudinal” axis or direction is parallel to the rails and in the direction of movement of the railway car on the track in any direction, and the manway is on the “top” of the tank car. The “transverse” or “lateral” axis or direction is in a horizontal plane perpendicular to the longitudinal axis and the rail. The term “inboard” means toward the center of the car, and may mean inboard in a longitudinal direction, a lateral direction, or both. Similarly, “outboard” means away from the center of the car. “Horizontal” is a plane parallel to the rails including the transverse and longitudinal axes, and “vertical” is the up-and-down direction. Extending “downward” means toward the ground.
Similarly to the prior art, gasket 200 according to the invention is received in circumferential groove 230 in manway cover 160.
In
Where the downward projection 122 contacts bolt lug 150 may be referred to as the “contact area.” The contact area is preferably greater than the cross sectional area of the bolt received in the slot 220. More preferably, the contact area is increased by a factor of 1.5. In this example, each of the six bolts has a diameter of ⅞ inch, and a cross sectional area (not counting thread profile) of 0.601 sq. in., and the contact area is 1.150 sq. in. The cover 160, including the stop, is generally a cast piece, but it is within the scope of the invention to attach a downward projection to an existing cover as a retrofit, by welding or other means known in the art.
In general, the gasket compression stop 122 is designed to allow the gasket to compress 25 to 60%. However, this is not to be deemed as limiting the invention. Polytetrafluoroethylene (PTFE) gaskets, EPDM rubber (ethylene propylene diene monomer (M-class) rubber) and nitrile (BUNA-N) rubber gaskets are also well known in the art. The thickness of a gasket made out of these materials, and the amount of deformation that the materials can withstand under compression, will vary. An advantage of the present invention is that the amount of compression can be controlled by machining the top of the eyebolt lug to maintain a specified distance between the top of the lug and the top of the nozzle wall. Alternatively, or in addition, the horizontal bottom surfaces on the downward projections 122 on the cover may be machined to achieve a specified clearance h1. Information concerning the elastic properties of gasket materials is readily available from the gasket manufacturers, so that the dimensions of the gasket compression stop can be developed accordingly. An advantage of the cover and nozzle system according to the invention is that dimensions h1 and h2 are design dimensions applicable to different systems to meet performance objectives.
An important aspect of the invention is that increasing the torque on the bolts after the predetermined amount of compression is achieved does not result in a greater stress localized around the eyebolt area. The stress on manway cover components may be evaluated using a pressure film, such as Medium Fuji Film Prescale pressure measurement film, which is a sheet comprising a polyester base layer coated with a color developing material, further layered with micro-encapsulated color forming material on top which breaks in response to pressure, thus reacting with color developing material to display color in proportion to pressure applied. A color chart is used to assess the compression stress developed at each position on the film.
Table 1 depicts the results of testing performed to estimate the pressure on a top surface of a gasket positioned on a manway nozzle during compression of the gasket with increasing torque applied on the bolts and to determine the effectiveness of the compression stops. The reported estimated pressure in this test is the highest pressure (darkest color) developed around the circumference of the nozzle. The uniformity of the pressure developed around the nozzle could also be evaluated by examining the pressure sensitive film. Of particular interest in this test was the pressure developed on the gasket in the location of the eye bolts.
To obtain this data, a pressure film was cut to size and placed over a ¼ inch thick rubber gasket on a manway cover nozzle. In Examples 1-7, according to the invention, a manway cover is provided with compression stops extending downwardly from manway cover ears positioned on opposed sides of each eye bolt. The opposed machined surfaces on the top of the bolt lug and the bottom of the compression stop were 0.08 inch apart before the bolts are tightened. In the Comparative Examples, 8-14, an otherwise identical conventional manway cover was employed for the test. Eye bolts were tightened by hand, using a torque wrench, in increments of 50 ft·lb to reach the maximum torque listed in the Table. Eyebolt tightening was done in both star (recommended) and circular (predicted field expedient) patterns for each load iteration, as noted in Table 1. In all of the Examples and Comparative Examples six eyebolts were used. Example 7 (according to the invention) and Comparative Example 14, bolts were overloaded immediately to 300 ft·lb (i.e., not in increments) to simulate overloading in the field.
Target gasket compression of 30-40% was achieved at just under 100 ft-lbs bolt torque; and compression stops engaged at just under 150 ft-lbs bolt torque. In Example 6, the bolts were all loaded beyond 150 ft·lbs. The consistent intensity pressure lines on the pressure film across the indexed eyebolt location showed that uniform pressure developed around the circumference of the nozzle and that gasket over-compression was prevented when compression stops were used according to the invention. Pressures developed on the gasket leveled off at higher amounts of torque applied to the bolts.
Another important aspect of the invention is ensuring that the cover and nozzle components of the system remain safely outside the plastic deformation regime and that the eye bolt is the first element to deform. With the compression stops according to the invention, torque applied to the eye bolts results in pressure distributed over a larger area after the compression stops contact the bolt lugs. Preferably, the contact area between the stops and the bolt lugs should be at least about as large as the cross-sectional area of the bolt itself. The increased contact area ensures that deformation occurs at the eye bolt before other parts.
In order to demonstrate this aspect of the invention, stresses encountered at critical points in the manway cover system may be modeled and analyzed using finite element analysis software, to determine maximum stress and maximum strain at the bolt, the manway cover ears on either side of the bolt, and the nozzle edge. Computer modeling provides an excellent understanding of relative stress and strain encountered at each part of the cover. The person of ordinary skill in the art will appreciate that an understanding of relative stresses and strains may be gained even where the absolute values of the stresses and strains do not exactly match the real world stresses and strains, as a result of limiting assumptions built into the software analysis.
A ratio of maximum stress to yield stress is referred to as a factor of safety (FoS). Comparisons of FoS results from load simulations show the cover and nozzle remaining outside the plastic deformation regime (FoS<1) at 450 ft·lb of torque applied to the bolts when the compression stops according to the invention are used, more than twice the maximum specified assembly torque. The increased area over which pressure is distributed also ensures that the bolt is the first element of the cover system to experience plastic deformation.
In the example of Table 2, finite element analysis software was used to determine the effect of applying torque equally to the eye bolts in a six-bolt cover at 250 ft·lb, 450 ft·lb, and 600 ft·lb, corresponding to axial loads of 26,167 lbf, 36,302 lbf, and 48,403 lbf, respectively, simulating full loading through the contact surfaces of the manway cover, nozzle, and eye-bolt. Bolt torque-force conversions were maximized by calculating with coefficient of friction under ideal lubricated conditions (all friction bearing surfaces).
Comparison of Factor of Safety results from simulated loading using a cover according to the invention shows both the cover and nozzle elements remaining safely outside plastic deformation regimes at 200 ft·lb, 250 ft·lb and 450 ft·lb, over twice the specified maximum assembly torques, while the eye-bolts were shown to be the weakest link in the assembly.
The description of the foregoing preferred embodiments is not to be considered as limiting the invention, which is defined according to the appended claims. The person of ordinary skill in the art, relying on the foregoing disclosure, may practice variants of the embodiments described without departing from the scope of the invention claimed. A feature or dependent claim limitation described in connection with one embodiment or independent claim may be adapted for use with another embodiment or independent claim, without departing from the scope of the invention.
