OLIVER HULL

DIALABLE ORIFICE FOR CONDENSATE SYSTEMS

THE AUTHOR

is a graduate of the United States Merchant Marine Academy from which he received a bachelor of science degree in marine engineering. Following graduation he was a sales engineer for Spirax-England and Sarcor-U, S . For the past flyteen years he has been responsible for Navy sales of heat exchangers and control valves for AERCO International, Inc. His current project involves the design and supply of external desuper- heater stations for the LHD-I class.

ABSTRACT

The use of an orifice goes back to the beginning of steam systems. Steam traps were developed to solve the problems of fluctuating boiler pressures and impurities in the condensate which caused clogging and erosion of orifices.

Steam generation has become more sophisticated and effi- cient. Studies to improve this efficiency have led to the conclu- sion that steam traps waste energy and when they fail, which they must, cause major steam losses. The U.S. Navy has deter- mined that significant fuel savings can be made by replacing traps with orifices.

Over the past few years, programs have been promulgated by the U.S. Navy to replace traps with orifices. Problems as to proper orifice size and strainer clogging have been experi- enced.

The dialable orifice was developed to correct these problems and to permit ease of orifice size selection. The dialable orifice incorporates the orifice disc, stop valves, strainer, and check valve in one compact unit, reducing installation costs and minimizing onboard repair parts.

INTRODUCTION

I n 1973, when distillate fuel cost $4.51 per barrel, it was estimated that fleet cost savings of fifteen million dollars per year were possible due to reduced fuel costs alone, by the reduction of steam leakage through replac- ing condensate removal traps with drain orifices. With current fuel costs of $20.58 per barrel, fifteen million dollars of savings in 1973 would translate into about 70 million dollars per year.

Costs associated with getting liquid water and non- condensible gases out of steam lines and steam using equipment are not limited to the cost of the fuel burned to heat steam which leaks through condensate draining devices. The cost of the hardware and of its initial in- stallation, the value of the space it occupies, costs asso- ciated with the weight of the equipment and its spare parts, and the costs of training maintenance personnel and of the time they must expend in actual preventive maintenance, monitoring, and repair on condensate drain equipment, all should appropriately be considered in comparing the costs of competing approaches to steam condensate eliminators.

158 Naval Engineers Journal, July 1987

AVAILABLE HARDWARE

In order to understand why, from cost considera- tions, one approach to removing condensed steam may be superior to another in a given application, a brief re- view of the methods by which liquid water and noncon- densible gases are eliminated from steam lines is in or- der.

Available hardware to keep steam in a pipeline or piece of steam using equipment, while allowing the es- cape of liquids and air-like gases, works on a few basic differences between the fluids involved. The difference in density between dry steam and liquid water at a given pressure is one basic way that a device can distinguish between the two phases to allow the escape of one while retaining the other. At 338 degrees F., and 115 psig, liq- uid water has a density of about 56 pounds per cubic foot, while steam is less than one-half of one percent as dense, at about .26 pounds per cubic foot. This differ- ence in density may be used to lift or position a float or may be taken advantage of due to the difference in mass flow which results when fluids of differing density are forced by a specified pressure difference to flow through a fixed restriction.

A second property which may be used to differentiate between steam and other fluids is the temperature at a given pressure. If the pressure is 115 psig, the tempera- ture of steam would have to be not less than 338 degrees F. At one atmosphere, or 14.7 psig, steam temperature must be at least 212 degrees F. A device employing a temperature sensing element, such as a vapor or liquid filled bellows, bi-metal strip, etc., can be made to close when the temperature approaches that of the steam to be retained, while opening when the temperature drops to a point that indicates the device has become filled with sub-cooled water or noncondensible gases.

A third property of water which may be taken advan- tage of in condensate drain devices is the tendency of condensate at or near saturation temperature to flash, or separate into two phases, one liquid and one vapor, as its pressure drops in flowing through a restriction. The large increase in specific volume of that portion of the hot condensate which flashes into steam as the pres- sure is reduced provides a basis for increasing the resis- tance to flow as the enthalpy of the fluid being drained increases.

For the purposes of this paper, condensate drain de- vices are classified as “moving parts” devices, such as traps, and “non-moving parts” devices, such as fixed restrictors.

TRAPS

Traps are moving parts devices. According to the op- erating principles employed, they may be categorized as

HULL DIALABLE ORIFICE

%ALL FLOAT--/ VALVE I i E A D l VALVE SEAT1

Figure 1. Float trap.

thermostatic, impulse, float, disc, float and thermo- static, inverted bucket, and various other names.

As shown in Figure 1, a float trap operates by the dif- ference in density between water and steam. High den- sity condensate will create a buoyant force on the float which, as it rises, opens a valve to allow the escape of condensate. The valve will open further until the float is at a position where an equilibrium is established be- tween condensate inflow and outflow.

Figure 2 shows another density difference style trap, the inverted bucket. In this case low density steam will float the bucket until condensate builds up to the point where the bucket “sinks,” pulling open a valve which allows the escape of air and condensate. This action is

:L F IOL

. E T

Figure 3. Thermostatic trap.

not generally a modulating one, and causes an on-off discharge from the trap under most circumstances.

Figure 3 illustrates a thermostatic trap which em- ploys a filled bellows that expands, closing off flow as the contents of the trap approach steam temperature, and which contracts, opening the valve to allow dis- charge of liquid water and noncondensibles as long as the temperature in the trap is below the saturation tem- perature of steam at the pressure involved.

An “F&T,” or float and thermostatic trap (not illus- trated), combines a float trap with a thermostatic trap, which allows venting of noncondensible gas that may collect above the condensate level maintained by the float activated valve.

A disc trap, shown in Figure 4, makes use of a flat valve disc which is held closed by steam pressure above the disc acting on a relatively large area, while the con- tained steam pressure acts upward on a small area. When the trap fills with cool condensate, heat transfer to the fluid above the disc is reduced, resulting in con- densation of the steam with a drop in pressure. The disc becomes unbdanced and lifts, allowing condensate out- flow until the trap again fills with steam and the disc is

~~~

Figure 2. Inverted bucket trap. Figure 4. Disc trap.

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HULL DIALARLE ORIFICE

forced shut by the downward impact pressure of escap- ing steam acting above the disc and low pressure caused by throttling escaping fluid between the disc and seat.

While as much ingenuity has been applied to the de- sign and manufacture of s t e m traps as to that of steam pressure and temperature regulating valves, which have both been produced for over a century in designs not much different from those which exist today, like steam valves, steam traps still are a major source of problems, breakdowns, and low reliability in steam utilizing equip- ment.

Many commonly used styles of traps inherently re- quire thermal losses from the body of the trap to the surrounding atmosphere in order to function. All of them depend on the maintenance of a tight seal between a disc or valve plug and its seat. How long a tight seal can be maintained is limited by the erosive action of the flow of water at relatively high pressure drops, which may be cavitating and/or flashing; by the flow of wet steam carrying very high velocity water droplets and dirt particles; and by the presence of dirt, rust flakes, miner- al deposits, weld slag, metallic wear debris, and other unavoidable contaminates which will deposit on or damage critical surfaces and eventually prevent the trap from closing, resulting in the discharge of large amounts of steam. In many cases, traps are also exposed to and suffer a limited life from the effects of water hammer, freezing, fatigue failure of flexible metal parts such as bimetal elements or bellows, and wear in mechanical linkages. Compared to a fixed restrictor, which will be discussed in the following section, traps are also heavy, bulky, and most types are orientation sensitive.

Compared to fixed restrictors, traps have limited reli- ability, high maintenance costs, and have high costs of energy loss either inherent in the design or associated with the tendency to fail open or leak excessively. In spite of these limitations, moving parts traps may be the best solution where the flow of condensate to the trap is at a constant pressure but varies over a very wide range under conditions where it is essential that condensate be rapidly and thoroughly drained from the system at all times.

FIXED RFSTRICTORS

Where the condensate formation rate does not vary over too wide a range, or where it tends to increase in a fixed relationship to the pressure drop across the con- densate eliminating device, a different approach to steam condensate removal has proven very cost effec- tive.

As can easily be seen by reference to standard sizing and flow calculation formulas, the mass flow rate of a liquid through a fiied restriction will typically be many times greater than the mass flow rate of the same fluid in the form of a vapor through the same restriction at the same pressure differential. For example, a 0.25 inch diameter orifice can pass about 5,800 pounds per hour of water, but only 241 pounds per hour of steam, assum- ing in both cases a 100 psig upstream pressure and an at- mospheric discharge pressure. This represents an ability

160 Naval Engineers Journal, July 1987

to pass 24 times as much condensate as steam through a device with no moving parts, no small passages to clog, and in its simplest implementation takes up virtually no space and weighs almost nothing compared to a trap with comparable capacity. With a smaller pressure drop, the condensate vs. steam discriminating ability of a simple orifice becomes even more favorable, with 43 times as much water as steam flowing through an orifice with an upstream pressure 1 psig above an atmosphere discharge.

Selecting a properly sized trap or orifice requires the upstream pressure, discharge pressure, and maximum condensate discharge rate needed to be considered. For a trap, oversizing would have little adverse effect if it worked in an ideal fashion. In the real world, however, the larger the trap capacity is compared to what is re- quired, the closer to the seat its valve will operate and the greater the chance is that the small clearance flow passages created will trap debris which can hold the trap open due to physical obstruction or cause damage to the seating surfaces. If a trap with a valve seat diameter of 0.50 inch operates under conditions where a 0.20 inch diameter orifice would create an equivalent resistance to flow, the valve in the trap will be positioned within 0.20 inch or less of its seat. Little comment is needed on the comparative sensitivity to malfunction due to particulate matter of a trap with a 0.02 wide flow path to that of an orifice with a 0.20 inch minimum dimension, when both are protected by a strainer which has 0.03 openings.

Like traps, fixed condensate flow restrictors are avail- able in a variety of forms.

A plate with a suitable sized hole in it is a simple form of fixed restrictor. Combined with spiral wound gasket assemblies on either side, such an orifice plate can be in- stalled between flanges. Incorporating a flat strainer disc in the upstream gasket provides a measure of pro- tection against clogging, although access to maintain the strainer requires breaking the bolted flange connection. The use of the above described method of draining high pressure steam mains on Navy ships goes back to 1969.

Manually adjustable valves may also be used as fixed restrictors to allow drainage of condensate. Unless care- fully selected, most valves used this way will be throt- tling close to the seat and will consequently have need- lessly small clearances in which dirt can be trapped. The possibility of tampering, resulting in the valve being left closed or open too far, is also a problem with this ap- proach. Sizing can be a problem also, unless the Cv vs. lift curve is accurately known for the valve in question. (C, is a measure of a valve’s capability to pass flow, and in the case of water is equal to the flow in GPM which results with a pressure differential of 1 psi across the valve. Lift is the percentage a valve is opened.)

The dialable orifice device has been designed to reduce the costs of condensate removed from drip legs and steam heating equipment operating at pressures up to 100 psig. As shown in Figures 5 and 6, it incorporates a maintainable strainer, a sharp edged orifice restriction, shut off valves, and an anti-backflow check valve into a compact body which requires only an inlet connection to the steam line to be drained and an outlet connection

HULL DIALABLE ORIFICE

~

Figure 5. Orifice disc.

to the condensate collection piping. A selection of ori- fices is provided for on a single disc, which may be ro- tated on initial installation to insert a precisely known and clearly marked orifice restriction into the conden- sate flow path. Initial results from field trials have pro- duced generally satisfactory results with improved func- tion of the heating equipment being drained reported.

DESIRABLE CHARACTERISTICS OF CONDENSATE REMOVAL DEVICES

For low cost condensate removal, the following char- acteristics of condensate removal devices are worth con- sideration and comparison.

1. The flow restricton should employ the largest mini- mum clearance possible for the flow rate to be han- dled. A round, sharp edged orifice is best in this re- spect.

2. A minimum of moving parts should be employed. Fixed restrictors require none. Traps require at least one moving part; some have many.

3. Thermal loss to the atmosphere should not be re- quired for the device to work. This is true of fixed re- strictors but not true of many types of traps.

4. The probability of fai1ure;modes which vent steam should be minimized. A fixed restrictor cannot “fail open” in the sense that a trap can.

5 . A minimum of additional fittings should be required, as they complicate installation and maintenance costs. Strainers, check valves, stop valves, etc., are usually part of a condensate removal installation and consequently may appropriately be made part of a single device.

6 . It should be possible to select a capacity close to that required, and to accurately know the capacity or de- gree of restriction of the device applied.

7. The device should remove not only sub-cooled con- densate, but also water at the saturation temperature. Traps which require 10 to 15 degrees of sub-cooling before condensate is discharged are not desirable in this respect.

8. A minimum number, size, and weight of replacement parts should provide for the support of a wide range of condensate drain installations.

CONCLUSION

A comparison of desirable characteristics for conden- sate removal devices with the characteristics of available hardware to do the job shows that the dialable orifice approach is worth careful evaluation in draining steam lines and equipment using steam for heating at conden- sate flow rates and steam pressures within its range.

The ultimate end of problems of condensate removal on Navy ships may be in sight.

There is a definite trend away from waste heat genera- tion for auxiliary systems since neither the DDG-51 Ar- leigh Burke class nor the FFG-7 Oliver Hazard Perry

INLET SHUT-OFF VALVE KNOB

!TRAINER---\ 7 STRAINER PLUG

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HULL DIALABLE ORIFICE

class will generate steam and the DD-963 Spruance class is to be converted to all electric. Even so, it appears like- ly that until the year 2000, most of the fleet will still use steam, and still have the problem of controlling conden- sate removal costs. If this is the case, it appears that the dialable orifice should be considered for both saving fuel and reducing maintenance.

REFERENCES

11) “Phaseout of High Pressure Steam Traps,” Lawrence L. Guzick, Naval Engineers Journal, April 1973, p. 5 .

162 Naval Engineers Journal, July 1987