Steam, Its Generation and Use Part 43

For pressures above 160 pounds, where superheated steam is used, all high pressure steam lines 4 inches and over should have solid rolled steel f.l.a.n.g.es and special upset lapped joints. In the manufacture of such joints, the ends of the pipe are heated and upset against the face of a holding mandrel conforming to the shape of the f.l.a.n.g.e, the lapped portion of the pipe being flattened out against the face of the mandrel, the upsetting action maintaining the desired thickness of the lap. When cool, both sides of the lap are faced to form a uniform thickness and an even bearing against f.l.a.n.g.e and gasket. The joint, therefore, is a strictly metal to metal joint, the f.l.a.n.g.es merely holding the lapped ends of the pipe against the gasket.

A special grade of soft flanging pipe is selected to prevent breaking.

The bending action is a severe test of the pipe and if it withstands the bending process and the pressure tests, the reliability of the joint is a.s.sured. Such a joint is called a Van Stone joint, though many modifications and improvements have been made since the joint was originally introduced.

The diameter and thickness of such f.l.a.n.g.es should be special extra heavy. Such f.l.a.n.g.es should be turned to diameter, their fronts faced and the backs machined in lieu of spot facing.

In lines other than given for pressures over 150 pounds, all f.l.a.n.g.es for wrought-iron pipe should be threaded. All threaded f.l.a.n.g.es for high pressure superheated lines 3 inches and under should be "semi-steel"

extra heavy. f.l.a.n.g.es for other than steam lines should be manufacturer's standard extra heavy.

Welded f.l.a.n.g.es are frequently used in place of those described with satisfactory results.

Fittings--For saturated steam under pressures up to 160 pounds, all fittings 3 inches and under should be screwed. Fittings 4 inches and over should have f.l.a.n.g.ed ends. Fittings for this pressure should be of cast iron and should have heavy leads and full taper threads. f.l.a.n.g.ed fittings in high pressure lines should be extra heavy, and in low pressure lines standard weight. Where possible in high pressure f.l.a.n.g.es and fittings, bolt surfaces should be spot faced to provide suitable bearing for bolt heads and nuts.

Fittings for superheated steam up to 70 degrees at pressures above 160 pounds are sometimes of cast iron.[78] For superheat above 70 degrees such fittings should be "steel castings" and in general these fittings are recommended for any degree of superheat. Fittings for other than high pressure work may be of cast iron, except where superheated steam is carried, where they should be of "wrought steel" or "hard metal".

Fittings 3 inches and under should be screwed, 4 inches and over f.l.a.n.g.ed.

f.l.a.n.g.es for pressures up to 160 pounds in pipes and fittings for low pressure lines, and any fittings for high pressure lines should have plain faces, smooth tool finish, scored with V-shaped grooves for rubber gaskets. High pressure line f.l.a.n.g.es should have raised faces, projecting the full available diameter inside the bolt holes. These faces should be similarly scored.

All pipe inch and under should have ground joint unions suitable for the pressure required. Pipe inch and over should have cast-iron f.l.a.n.g.ed unions. Unions are to be preferred to wrought-iron couplings wherever possible to facilitate dismantling.

Valves--For 150 pounds working pressure, saturated steam, all valves 2 inches and under may have screwed ends; 2 inches and over should be f.l.a.n.g.ed. All high pressure steam valves 6 inches and over should have suitable by-pa.s.ses. All valves for use with superheated steam should be of special construction. For pressures above 160 pounds, where the superheat does not exceed 70 degrees, valve bodies, caps and yokes are sometimes made of cast iron, though ordinarily semi-steel will give better satisfaction. The spindles of such valves should be of bronze and there should be special necks with condensing chambers to prevent the superheated steam from blowing through the packing. For pressures over 160 pounds and degrees of superheat above 70, all valves 3 inches and over should have valve bodies, caps and yokes of steel castings.

Spindles should be of some non-corrosive metal, such as "monel metal".

Seat rings should be removable of the same non-corrosive metal as should the spindle seats and plug faces.

All salt water valves should have bronze spindles, sleeves and packing seats.

The suggestions as to f.l.a.n.g.es for different cla.s.ses of service made on page 311 hold as well for valve f.l.a.n.g.es, except that such f.l.a.n.g.es are not scored.

Automatic stop and check valves are coming into general use with boilers and such use is compulsory under the boiler regulations of certain communities. Where used, they should be preferably placed directly on the boiler nozzle. Where two or more boilers are on one line, in addition to the valve at the boiler, whether this be an automatic valve or a gate valve, there should be an additional gate valve on each boiler branch at the main steam header.

Relief valves should be furnished at the discharge side of each feed pump and on the discharge side of each feed heater of the closed type.

Feed Lines--Feed lines should in all instances be made of extra strong pipe due to the corrosive action of hot feed water. While it has been suggested above that cast-iron threaded f.l.a.n.g.es should be used in such lines, due to the sudden expansion of such pipe in certain instances cast-iron threaded f.l.a.n.g.es crack before they become thoroughly heated and expand, and for this reason cast-steel threaded f.l.a.n.g.es will give more satisfactory results. In some instances, wrought-steel and Van Stone joints have been used in feed lines and this undoubtedly is better practice than the use of cast-steel threaded work, though the additional cost is not warranted in all stations.

Feed valves should always be of the globe pattern. A gate valve cannot be closely regulated and often clatters owing to the pulsations of the feed pump.

Gaskets--For steam and water lines where the pressure does not exceed 160 pounds, wire insertion rubber gaskets 1/16 inch thick will be found to give good service. For low pressure lines, canvas insertion black rubber gaskets are ordinarily used. For oil lines special gaskets are necessary.

For pressure above 160 pounds carrying superheated steam, corrugated steel gaskets extending the full available diameter inside of the bolt holes give good satisfaction. For high pressure water lines wire inserted rubber gaskets are used, and for low pressure f.l.a.n.g.ed joints canvas inserted rubber gaskets.

Size of Steam Lines--The factors affecting the proper size of steam lines are the radiation from such lines and the velocity of steam within them. As the size of the steam line increases, there will be an increase in the radiation.[79] As the size decreases, the steam velocity and the pressure drop for a given quant.i.ty of steam naturally increases.

There is a marked tendency in modern practice toward higher steam velocities, particularly in the case of superheated steam. It was formerly considered good practice to limit this velocity to 6000 feet per minute but this figure is to-day considered low.

In practice the limiting factor in the velocity advisable is the allowable pressure drop. In the description of the action of the throttling calorimeter, it has been demonstrated that there is no loss accompanying a drop in pressure, the difference in energy between the higher and lower pressures appearing as heat, which, in the case of steam flowing through a pipe, may evaporate any condensation present or may be radiated from the pipe. A decrease in pipe area decreases the radiating surface of the pipe and thus the possible condensation. As the heat liberated by the pressure drop is utilized in overcoming or diminis.h.i.+ng the tendency toward condensation and the heat loss through radiation, the steam as it enters the prime mover will be drier or more highly superheated where high steam velocities are used than where they are lower, and if enough excess pressure is carried at the boilers to maintain the desired pressure at the prime mover, the pressure drop results in an actual saving rather than a loss. The whole is a.n.a.logous to standard practice in electrical distributing systems where generator voltage is adjusted to suit the loss in the feeder lines.

In modern practice, with superheated steam, velocities of 15,000 feet per minute are not unusual and this figure is very frequently exceeded.

Piping System Design--With the proper size of pipe to be used determined, the most important factor is the provision for the removal of water of condensation that will occur in any system. Such condensation cannot be wholly overcome and if the water of condensation is carried to the prime mover, difficulties will invariably result.

Water is practically incompressible and its effect when traveling at high velocities differs little from that of a solid body of equal weight, hence impact against elbows, valves or other obstructions, is the equivalent of a heavy hammer blow that may result in the fracture of the pipe. If there is not sufficient water in the system to produce this result, it will certainly cause knocking and vibration in the pipe, resulting eventually in leaky joints. Where the water reaches the prime mover, its effect will vary from disagreeable knocking to disruption.

Too frequently when there are disastrous results from such a cause the boilers are blamed for delivering wet steam when, as a matter of fact, the evil is purely a result of poor piping design, the most common cause of such an action being the pocketing of the water in certain parts of the piping from whence it is carried along in slugs by the steam. The action is particularly severe if steam is admitted to a cold pipe containing water, as the water may then form a partial vacuum by condensing the steam and be projected at a very high velocity through the pipes producing a characteristic sharp metallic knock which often causes bursting of the pipe or fittings. The amount of water present through condensation may be appreciated when it is considered that uncovered 6-inch pipe 150 feet long carrying 3600 pounds of high pressure steam per hour will condense approximately 6 per cent of the total steam carried through radiation. It follows that efficient means of removing condensation water are absolutely imperative and the following suggestions as to such means may be of service:

The pitch of all pipe should be in the direction of the flow of steam.

Wherever a rise is necessary, a drain should be installed. All main headers and important branches should end in a drop leg and each such drop leg and any low points in the system should be connected to the drainage pump. A similar connection should be made to every fitting where there is danger of a water pocket.

Branch lines should never be taken from the bottom of a main header but where possible should be taken from the top. Each engine supply pipe should have its own separator placed as near the throttle as possible.

Such separators should be drained to the drainage system.

Check valves are frequently placed in drain pipes to prevent steam from entering any portion of the system that may be shut off.

Valves should be so located that they cannot form water pockets when either open or closed. Globe valves will form a water pocket in the piping to which they are connected unless set with the stem horizontal, while gate valves may be set with the spindle vertical or at an angle.

Where valves are placed directly on the boiler nozzle, a drain should be provided above them.

High pressure drains should be trapped to both feed heaters and waste headers. Traps and meters should be provided with by-pa.s.ses. Cylinder drains, heater blow-offs and drains, boiler blow-offs and similar lines should be led to waste. The ends of cylinder drains should not extend below the surface of water, for on starting up or on closing the throttle valve with the drains open, water may be drawn back into the cylinders.

TABLE 64

RADIATION FROM COVERED AND UNCOVERED STEAM PIPES

CALCULATED FOR 160 POUNDS PRESSURE AND 60 DEGREES TEMPERATURE

+---------------------------------------------------------------------+ |+------+---------------------------+----+----+----+-----+-----+-----+| || | | | | | | | || || Pipe | |1/2 |3/4 | 1 |1-1/4|1-1/2| || ||Inches| Thickness of Covering |inch|inch|inch|inch |inch |Bare || |+------+---------------------------+----+----+----+-----+-----+-----+| || |B. t. u. per lineal foot | | | | | | || || | per hour |149 |118 | 99 | 86 | 79 | 597 || || |B. t. u. per square foot | | | | | | || || | per hour |240 |190 |161 | 138 | 127 | 959 || || 2 |B. t. u. per square foot | | | | | | || || | per hour per one degree | | | | | | || || | difference in temperature|.770|.613|.519|.445 |.410 |3.198|| |+------+---------------------------+----+----+----+-----+-----+-----+| || |B. t. u. per lineal foot | | | | | | || || | per hour |247 |193 |160 | 139 | 123 |1085 || || |B. t. u. per square foot | | | | | | || || | per hour |210 |164 |136 | 118 | 104 | 921 || || 4 |B. t. u. per square foot | | | | | | || || | per hour per one degree | | | | | | || || | difference in temperature|.677|.592|.439|.381 |.335 |2.970|| |+------+---------------------------+----+----+----+-----+-----+-----+| || |B. t. u. per lineal foot | | | | | | || || | per hour |352 |269 |221 | 190 | 167 |1555 || || |B. t. u. per square foot | | | | | | || || | per hour |203 |155 |127 | 110 | 96 | 897 || || 6 |B. t. u. per square foot | | | | | | || || | per hour per one degree | | | | | | || || | difference in temperature|.655|.500|.410|.355 |.310 |2.89 || |+------+---------------------------+----+----+----+-----+-----+-----+| || |B. t. u. per lineal foot | | | | | | || || | per hour |443 |337 |276 | 235 | 207 |1994 || || |B. t. u. per square foot | | | | | | || || | per hour |196 |149 |122 | 104 | 92 | 883 || || 8 |B. t. u. per square foot | | | | | | || || | per hour per one degree | | | | | | || || | difference in temperature|.632|.481|.394|.335 |.297 |2.85 || |+------+---------------------------+----+----+----+-----+-----+-----+| || |B. t. u. per lineal foot | | | | | | || || | per hour |549 |416 |337 | 287 | 250 |2468 || || |B. t. u. per square foot | | | | | | || || | per hour |195 |148 |120 | 102 | 89 | 877 || || 10 |B. t. u. per square foot | | | | | | || || | per hour per one degree | | | | | | || || | difference in temperature|.629|.477|.387|.329 |.287 |2.83 || |+------+---------------------------+----+----+----+-----+-----+-----+| +---------------------------------------------------------------------+

Covering--Magnesia, canvas covered.

For calculating radiation for pressure and temperature other than 160 pounds, and 60 degrees, use B. t. u. figures for one degree difference.

Radiation from Pipes--The evils of the presence of condensed steam in piping systems have been thoroughly discussed above and in some of the previous articles. Condensation resulting from radiation, while it cannot be wholly obviated, can, by proper installation, be greatly reduced.

Bare pipe will radiate approximately 3 B. t. u. per hour per square foot of exposed surface per one degree of difference in temperature between the steam contained and the external air. This figure may be reduced to from 0.3 to 0.4 B. t. u. for the same conditions by a 1 inch insulating covering. Table 64 gives the radiation losses for bare and covered pipes with different thicknesses of magnesia covering.

Many experiments have been made as to the relative efficiencies of different kinds of covering. Table 65 gives some approximately relative figures based on one inch covering from experiments by Paulding, Jacobus, Brill and others.

TABLE 65

APPROXIMATE EFFICIENCIES OF VARIOUS COVERINGS REFERRED TO BARE PIPES +--------------------------------+ |+-------------------+----------+| || Covering |Efficiency|| |+-------------------+----------+| ||Asbestocel | 76.8 || ||Gast's Air Cell | 74.4 || ||Asbesto Sponge Felt| 85.0 || ||Magnesia | 83.5 || ||Asbestos Navy Brand| 82.0 || ||Asbesto Sponge Hair| 86.0 || ||Asbestos Fire Felt | 73.5 || |+-------------------+----------+| +--------------------------------+

Based on one-inch covering.

The following suggestions may be of service:

Exposed radiating surfaces of all pipes, all high pressure steam f.l.a.n.g.es, valve bodies and fittings, heaters and separators, should be covered with non-conducting material wherever such covering will improve plant economy. All main steam lines, engine and boiler branches, should be covered with 2 inches of 85 per cent carbonate of magnesia or the equivalent. Other lines may be covered with one inch of the same material. All covering should be sectional in form and large surfaces should be covered with blocks, except where such material would be difficult to install, in which case plastic material should be used. In the case of f.l.a.n.g.es the covering should be tapered back from the f.l.a.n.g.e in order that the bolts may be removed.

All surfaces should be painted before the covering is applied. Canvas is ordinarily placed over the covering, held in place by wrought-iron or bra.s.s bands.

Expansion and Support of Pipe--It is highly important that the piping be so run that there will be no undue strains through the action of expansion. Certain points are usually securely anch.o.r.ed and the expansion of the piping at other points taken care of by providing supports along which the piping will slide or by means of flexible hangers. Where pipe is supported or anch.o.r.ed, it should be from the building structure and not from boilers or prime movers. Where supports are furnished, they should in general be of any of the numerous sliding supports that are available. Expansion is taken care of by such a method of support and by the providing of large radius bends where necessary.

It was formerly believed that piping would actually expand under steam temperatures about one-half the theoretical amount due to the fact that the exterior of the pipe would not reach the full temperature of the steam contained. It would appear, however from recent experiments that such actual expansion will in the case of well-covered pipe be very nearly the theoretical amount. In one case noted, a steam header 293 feet long when heated under a working pressure of 190 pounds, the steam superheated approximately 125 degrees, expanded 8 inches; the theoretical amount of expansion under the conditions would be approximately 9-35/64 inches.