1965_aswegen_boiler mountings and control

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78 Proceedings of The South African Sugar Teclinologists' Association-Marc11 1965

BOILER MOUNTINGS AND CONTROL EQUIPMENT FORPROCESS STEAMBy W. G. AN ASWEGEN $:

Boiler Mountings is the overall description appliedto the valves and items of equipment that not onlycontrol the services to and from the boiler, but alsoindicate what is happening to the fluid inside theboiler, give warning should dangerous cond itions ariseand finally operate to ensure the safety of plant andpersonnel.

These mountings must be carefully selected fromproven designs as the availability of any boiler isdependent on their trouble-free operation. T heir con-struction should be robust (without over-designing),all materials carefully selected and quality controlledduring all phases of manufacture. Finally, the design

should enable maintenance to be carried out simplyand effectively with a minimum of expense.

Ap art f rom designing valves from a purely technicalaspect to meet certain duties, the manufacturer mustalso pay attention to regulations which govern thedesign and use of boiler appurtenances. These regu-lations vary from one country to the next and anypurchaser should obtain the assurance that all fittingscomply with the current local regulations. This pointis particularly pertinent at the moment as our Fac-tories, Machinery and Building Work Act has beenrevised fairly recently. Points such as feed checkvalves, main steam stop valves, inspectors' test con-nections and others bear investigation. It is assumed

that you are all familiar with these local requirementsbut it should be noted that there are three otherspecifications which can be referred to for additionalinformation. British Standard Specification 759 dealswith valves, gauges and fittings, B.S. 806 covers,"Ferr ous Pipes and Piping Installations", and B.S. 10gives revised ratings for flanges. Although B.S. 806refers specifically to piping systems, the reco mmend a-tions for design conditions f or piping must be observedwhen choosing the valves to operate in that system.

The reason for mentioning B.S. 10 is that with thegeneral uprating of Mild Steel and Copper flanges,valve manufacturers have had to investigate theirdesigns to see whether their various valve componentsare strong enough to be uprated to the same extent.It will be appreciated that this involves an immenseam oun t of w ork and it will still be a little while beforethe impact of the flange uprating is absorbed by thevalve industry. Where body components do not meetthe higher ratings they must either be stiffened up oralternatively equipped with a lower flange rating.

This feature bears investigation when consideringnew or replacement plant. If, in the past, you havebeen in th e habit of specifying a 350 p.s.i. rating valvewith Table "J" flanges for working conditions of 350p.s.i. and 600" F., you will now find that the Table"H" flange is uprated to 390 p.s.i. at 600" F. If thevalve manufacturer has found his general design satis-factory, he will now be offering his so called "250pattern" Table "H" valve for this duty at a consider-able saving to the user.

Briefly the effect of the flange uprating has notaffected Mild Steel valves operating at the duty tem-perature of 800" F. where, for example, Tables "H"and "J" remain at 250 and 350 p.s.i., respectively. Theuprat ing mainly affects the temperature range between450" F. and 800" F. and roughly speaking the pressurerating between 800" F. and 450" F. is doubled. Below450" F. the rating remains constant.

Cast Iron valves are relatively unaffected by thesechanges, but tables "B" and "C" have been dropped .Copper-Alloy valves have benefited slightly- hat isto say if the body has been uprated with the flange.

Steam plant users should m ake use of these increasedratings for flanged valves but only with the permissionof the valve supplier. Most of the catalogues currentlyin use will not yet have caught up with the upratingsand it would be very ill-advised to assume this.

Where the South African Regulations are at variancewith these other specifications, the former will apply.

'Hylif' Safety Valve



Proceeditlgs of' The Sorrtli Africatz Sugar Tecl~nologists'Associution-

Whereas, in certain industries, designs change everyyear to catch the market, you will not find anythinglike the same change if you consider the valve indus-try- specially in the ran ge up to 600 p.s.i. This doesnot mean that changes never occur. These changesare more in the form of developments resulting after

years of operation and improvements in new materialsand manufacturing skills.

No reputable valve manufacturer will risk an un-

proven design for general service. Any new design is

thoroughly tested in the Works Laboratory and thenfield tested on some installation under close observa-

tion to iron out details. It is not always the largecomponents such as body castings or spindles that

give trouble, but the smaller items, such as gland

packings and lid joints that sometimes prove critical.

Most manufacturers buy out their packings (withperhaps some modification to suit individual designs)and these are given the same exhaustive tests. As ageneral rule- f you have purchased a high classproduct you will not achieve anything by experimen-ting with other makes of packings, jointing andlubricants. In an emergency it is usually possible to

obtain a substitute material but this should be replacedwith Makers' standard as soon as possible.

There are exceptions to the rule but these cases canusually be traced to different fluid conditions, opera-ting procedures, lack of maintenance and others.

While on the subject of development, some com-ments on the parallel slide valve should prove inter-esting. This valve basically comprises two lapped

discs which are spring-loaded onto two opposinglapped seats. The spring merely serves to keep thefaces in close contact. thus wiping away any d irt tha tm igh t lo dge o n t he surfaces. ~ h &hed is; is movedinto the lap position, the differential pressure forcesthe dow nstream disc harder onto the seat, thus sealingoff the flow. If the flow is reversed, the opposite discseals off. The body space is therefore always underpressure and it is not possible to carry out any glandrepairs.

From this full bore straight-through valve has de-veloped a venturi pattern parallel slide valve with theseat bore equal to 2/3rds of the pipe bore. This meansthat the relative area is slightly less than half the pipearea (4/9ths) which will cause an increase in velocitythrough the bore of the valve.

When the valve is in the fully open position, aneye piece is drawn up to bridge across the seat gapthus completing the converging-diverging nozzle. Theresultant pressure drop is negligible.

Other advantages are less load on the spindle dueto the smaller area, reduced valve travel resulting ina more compact valve and protection of the faces bythe eye piece in the fully open position.

One of the most important claims for the parallelslide design is the ability of the valve to remain tight

under varying conditions of temperature and pressure.The same claim cannot be made for the globe orwedge valve which can be affected by physical changesin dimensions under temperature and pressure.

This sealing feature is used on a num ber of differentitems of equipment such as steam trap seats, high andlow whistle alarm seats, water gauge arm seats, blow-down and drain valve seats. In some cases only asingle disc and seat is used if the flow is in on e directiononly.

The venturi pattern valves are sized by their borematching to the nominal pipe size and not by thereduced throat bore.

Anoth er d evelopm ent of the. venturi valve is theVEE notch seat. Instead of providing the usual ringseat on the downstream side, a flat faced lapped discis fitted which has a VEE notch down the verticalaxis. This gives excellent regulating qualities for h andcontrol to by-pass some automatic controller whichis temporarily ou t of commission.

Water gauges deserve some mention as their con-tinuous and accurate functioning is vital. On thehigher pressure boilers used in Sugar Mill practicetoday, the plate glass type indicator with automaticsafety ball sealing in top and bottom arms is prac-tically standard. At these higher saturation tempera-

tures the glass is rapidly attacked and must be pro-tected by a thin film of mica usually .015 n. thick.



80 Proceedings of The South African Sugar Technologists' Association-March 1965

A variation of this type of gauge was tried by aContinental niaiiufacturer some years ago. A layer ofniica some 3/32 in. thick with no glass backing wasclamped between steel plates. This was developed forvery high pressures where the dissolving action wasvery aggressive and the thermal shock due to ambient

conditions and expansion and contraction of tlie steelcage was sufficient to fracture the glass.

It plate glass type water gauges are placed indraughty positions they invariably give more troubledue to thcrmal shock. The fluctuations in level of theboiler water also cause thermal shock to the gauge.Gauges should preferably be placed as close to theboiler pads a s possible to reduce h eat losses an d main-tain tlie whole unit at an even temperature. If thereare interconnecting pipes they should be of adequatesize with tlie water arm well lagged and the steam ar mlightly lagged and sloping down towards the gauge.This will encourage a flow of condensate to the gaugewhich will help maintain an even temperature.

A later development in water gauges is the mul tiportgauge which consists of a row of small openingsarranged in a vertical line. By making use of thedifferent degrees of light refraction through steam andwater, light is projected through strips of colouredglass which projects the water s p a~ e~ b lu end the steamspace red. These smaller individual glasses with micaprotection are less susceptible to thermal shock.

By way of interest, the latest development in remo tewater level indicatory equipment takes the form of atelevision unit which comprises a scanning unitmounted in fron t of the water gauge linked to a c athoderay tube fitted to the boiler control panel.

Before going onto the process side, let us briefly

consider the continuous blowdown system of keepingthe total dissolved solids in check. The immediateadvantage of this system is that the maximum con-centration of solids is the average concentration,whereas with intermittent blowdown the maximumconcentration can be far greater than the average.

A further advantage is that because the water isbeing discharged at a steady controlled rate its heatcontent can be m ore readily tapped. It is recommendedthat for boiler plant at, say, 350-450 p.s.i. the firststage of heat recovery is by flashing the blowdown ina su itable vessel and using this flash steam for process.The residual condensate will now have an even higherconcentration of solids and must be discharged towaste by means of a float valve or inverted "U" seal.As this conden sate still has sensible heat, the dischargecan take place through a heat exchanger.

In this country, with cheap coal, there is a limit tothe expenditure of installing equipment to recover theheat from blown down water. Apart from controllingthe solids more accurately, a one-stage flash unit andthen to waste would suffice.

"Lapping a Large Valve Disc"



Proceedings of The South African Sugar Technologists' Association-March 1965 8 1

Let us now consider process steam. Careful plan-ning is necessary to balance the requirements of steamfor power and process, the latter being in greaterdemand.

Although pass-out turbines are generally employedfor power generation and some of the larger drives,

it is necessary to install pressure reducing valves toprovide the considerable L.P. process steam required.

In earlier days superheat was not generally usedand the process of pressure reduction had the effectof drying the steam. In theory it would appear thata slight degree of superheat is possible provided youcommence with that mythical fluid called "dry satu-rated steam". Under actual Mill conditions this "dry"steam can be anything up to 5 % wet and the evapo-ration of this moisture will absorb any gain by ex-panding a t consta nt heat which is what happens whenthe steam is not required to do work. However, withthe increased tendency for power generation by means

of steam turbines, pressures and temperatures havecrept up and operating conditions of 450 p.s.i. and750" F. are not uncommon. When augmenting pass-out and exhaust steam under these conditions, it isfound that the overall temperature after pressurereduction only falls off slightly and the resulting L.P.steam is left with an even higher degree of superheat.

Experiments and practice have shown tha t say 50"F.of superheat is quite acceptable for distribution to

process as it does not interfere with the heat transferand will, in fact, improve the quality of the exhaustor pass-out steam with which it is mixed. Superheatof several hundred degrees F. cann ot be permitted asthe rate of heat transfer is approximately 1/100 thatof saturated steam. Under these conditions it is

necessary to install desuperheating equipment down-stream from the reducing valve to control the tem-perature to say 50" F. above saturation temperature.Another prac tical reason for n ot desupzrheating rightdown to saturation point - specially if you areblending the s team- s the difficulty of ensu ring tha tno excess water is injected under widely fluctuatingdemands. If you aim to retain some slight degree ofsuperheat, you will allow the plant more flexibility.

A highly efficient method of desuperheating is toinject water into the flow of superheated steam in theform of a fine spray so as to absorb the necessarylatent heat from the steam, reducing its temperature.This water must be pure as it will blend with the

steam flow to increase the total quantity of steam.Furthermore, the temperature should also be close tothe saturated temperature corresponding to the L.P.condition. In this way there will be no time lag whilethe injected water absorbs sensible heat before itreaches the point when it is ready to flash into steam.In order to obtain a fine spray it is also necessary toinject water at a pressure of say 150 p.s.i. above theinterna l pressure in the desuperheater. A con stant flow

"Lapping Small Valve Discs"



Proceedings of Tfie South African Siigar Teclmologists' Association-Marc11 1965

of spray water from a centrifugal pump or feed rangeis preferable to the pulsating delivery from a plunger

Pump.The amount of desuperheating water required is a

function of the total heat loss necessary to drop thetemperature of the steam to the desired value. The

total heat loss can be expressed by (H inlet-controlled) x flow in Ib./hr., where the values of Hare the total heat figures in BTU7s/lb.at the inlet andcontrolled conditions.

This quantity of heat must be absorbed by "X"lb. of water per hour, which must take in sensibleheat as well as latent heat, plui superheat, finally toreach the controlled temperature. In solving for "X"it will be noted that the colder the injected water theless quantity is required, but as previously stated, weare interested in rapid and efficient heat transfer,hence the requirement of water as close to saturationpoint. Another practical reason is the severe thermalshock which would be set up by injecting cold water.

Not only will this result in leaking flange joints, butit can also cause fatigue of the pipe.

The heat which the water will absorb can thereforebe taken as the total heat a t the controlled temperature(H controlled) minus the sensible heat of the waterat the injection temperature (h controlled).

(H inlet)- H controlled)i.e., X inlb./hr.= x steam

(H controlled)-(h controlled)flow Ib./hr.

Reverting to pressure controllers, these are availablein all shapes and sizes from small direct acting dia-phragm types to large double beat electrically orpneumatically controlled valves capable of reducingfrom full boiler pressure to process pressure in onestage. Certain requirements and limitations are com-

mon to all makes of reducing valves and should beobserved in order to obtain the maximum satisfaction.

In general:

(a) Small direct-acting or relay reducing valves,say, 2 in. and below should be protected bymeans of a strainer.

(b ) Entrained moisture is the enemy of all reducingvalves and this sho uld be removed by a separa-tor and traps. In smaller installations themethod of tapping off the steam will achievethe same result.

(c) The pipe sizes upstream and downstream of a

reducing valve should be adequate to conveythe maximum steam flow without throttling.All throttling should be restricted to the valveotherwise it must contend with a varying pipecharacteristic. This is more important on thelow pressure side. This pipe characteristic can,however, be overruled by obtainin g the lowpressure impulse from a point close to wherethe steam is required.

(d) If reducing valves are positioned in branchpipes leading out of a main, the valve should

"Method of Lapping Seat in valve Body"



Proceedings of The South African Sugar Technologists' Association-

not be placed closz to the main. The swirlingof the steam as it enters the branch can causedamage to the valve seat by causing it to spin.

(e) Reducing valves which are in continuous usemust receive regular maintenance especiallythose with spindles passing through a gland tosome outside source of control. Seats shouldalso be checked for leakage because once thesteam starts cutting at a point, the wear israpid. The valve need not be stripped downfor this purpose as it is possible to detect aleakage hiss when the valve is shut. Also therate of build-up of pressure in the L.P. systemunder no load conditions will also providesome indication. However, it must be realisedthat very few, if any, makers will claim abso-lute shut-off for a reducing valve and a smalldegree of passing mu st be tolerated.

(f) Reducing valves must not be oversized as this

will lead to wire drawing and cutting of theseats under low load condition. In cases wherelow demands must be supplied for long periodswith some peaks, two valves in parallel areused with different impulse settings.

The above are a few of the many points to bear inmind when specifying and installing reducing equip-ment.

If any item of equipment in the low pressure systemis not designed for the maximum inlet pressure, safetyvalves are necessary. When dealing with boiler moun-tings this important item was omitted as it couldconveniently be discussed at this stage. Except forspecial low pressure and exhaust relief valves, the

identical valve as used for boiler mountings will befound on low pressure piping systems. Their dischargecapacities when used as Boiler Mou ntings conform t othe em pirical formulae as listed in BS 759. When usedto protect piping systems different rules apply butbasically the discharge is again a func tion of the abso-lute set pressure with a correction factor for the degreeof superheat. Most manufacturers publish charts lis-ting the capacity in lb./hr.

A m ost desirable feature of a safety valve is a con-trol on the blowdown. The blowdown of a safetyvalve is the difference in pressure between the setpressure and the reseating pressure. In an emergency

adequate relief must be supplied but by the same tokenno unnecessary waste of steam energy must occur.BS 759 makes certain suggestions in this regard.

Acknowledgments

The author wishes to thank Stewarts & Lloyds ofS.A. Ltd.; and Hopkinsons Ltd., of Huddersfield,England, for permission to publish this paper.

"Lapping small Parallel-slide and 'Uniflow' Valves"



84 Proceedings o f The South African Sugar Technologists' Association-Marc11 1965

Mr. Gunn (in the chair): In continuous blow down,if the dissolved solids in the boiler are severe, howlong will the valves last? And also, how long does thelapping plate, with its five thousandths of an inchgrooves, last ?

Mr. Aswegen: If there are a lot of particles goingthrough a narrow orifice you are bound to get somewear. Even stainless steel would show wear.

An orifice plate can be placed in series with a valveand this will destroy pressure, the valve giving thecontrol. An orifice discharging from a four hundredand fifty p.s.i. boiler down to atmosphere destroys alot of energy and if the valve seat has to take it all itwon't last very long.

The lapping plates do wear but when they are beinggrooved tallow is rubbed well into the surface andemery powder is sprinkled on top.

On occasions when I have not had a lapping plateI have taken a sheet of plate glass $" thick and coatedthat with grinding compound. It has damaged the

glass slightly but the wear has not been very rapid.With average maintenance the life of a plate would

be several years before it would have to be recon-ditioned.

Mr. Saunders: There are now machines that do thelapping without requiring arduous manual labour.

Mr. van Aswegen: The type of machine you referto is used mainly where the valves are welded in ,

because it is very difficult and expensive to remove thevalve.

1965_aswegen_boiler mountings and control

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