Using ball valves in control applications They can provide performance improvements for less cost

F. W. Brcken, B-Controls, Desseldorf, Germany

Ball control valves, the most important design of the quarter-turn control valve (QTCV) family, have become important control hardware for the process industries. Market analysts report that they represent 14.5% of all control valves installed in petrochemical process units. Furthermore, it is reported that ball control valve packages are increasing 4.5% per year in market share compared to linear control valve sales. Several technological developments are responsible for this growth.

The most important drivers for this market are:

Specific cost of $/cv value Lower cost per valve size and/or control loop Lower cost for tight shut-off requests.

In total, it is a control valve concept with commercial and technical advantages.

The instrument engineer confronted with such statements does not have a sound basis for deciding on QTCVs unless comprehensive tools are made available. In the past, high technology in the field of linear control valves has been developed and users have been educated in the technology. For the hardware of leading globe valve producers, users are enjoying a sound basis for sizing and specifying control valves.

This article will develop confidence and introduce tools for the ball control valve product family. Similar to efforts of a few other QTCV producers, methods for commercial comparison and technical concepts will make this product technology worth considering. Engineering basics for ball control valves, the complete and most advanced technology for valve sizing and sound pressure level (SPL) prediction of this valve type, plus serious solutions for reducing SPL are presented.

Basic design. The basic design involves the concepts of floating and trunion-mounted ball valves. Both designs are available in metal-seat technology, plus elastomer-seated units if needed. Present metal-seat technology in ball control valves provides tight shut-off capabilities far above the criteria of soft-seated control valve leak rate standards. The metal-seated ball control valve can hold a tightness equal to 1 x 10 - 5 mbar x lt/sec (about 1 ccm over 24 hr). Generally, a control valve does not need to be a shut-off valve, but if the feature comes almost free, it is not a disadvantage. Important to mention is that temperature is rarely a problem.

The decision between a floating (Fig. 1) or trunion-mounted (Fig. 2) ball depends mainly on the differential pressure, p, the valve has to open or close against. Relatively high p loads are acting on the large ball radius as a torque generator of the ball. The result is a high demand for actuator torque equal to a cost increase for the total unit.

Fig. 1. Floating-ball control valve.

Fig. 2. Trunion-mounted ball control valve.

The trunion-mounted ball is supported at upper and lower axes, operating on a relatively small bearing radius. Since this results in much smaller torque requirements, the more expensive trunion-mounted design, operating under high p, can be much lower in cost than the floating-ball concept.

No general rule exists for deciding between the floating and the trunion-mounted ball. Assuming the inner valve parts and materials would tolerate both concepts, the decision will be based on total valve plus actuator cost. The extreme situation can be that a small valve with very high p is most economical as a floating-ball design. Or vice versa, a larger valve with relatively low p should be a trunion-mounted design because of the torque requirements and actuation.

Ball control valve parameters. Fig. 3 shows the most important coefficients.

Fig. 3. Ball control valve characteristics.

The valve characteristic is approximately equal-percentage. This is the result of the inner valve without any modification or addition, such as characterized windows. Following the standards for the "inherent flow characteristic and rangeability of control valves," it can be stated: If the manufacturer shows graphically or in tabular form the values of flow coefficients and sizing factors vs % of opening, the characteristic name is of secondary importance.

Ball control valve rangeability and maximum flow coefficient are much larger than is generally required. A serious relativity can be made without loosing credits to the linear valve family.

Inherent flow characteristic. The characteristic of a ball control valve is shown in Fig 4. Furthermore, lines are plotted for permissible tolerance of the published valve characteristic (approximately equal-percentage), equal to (10 x (1/cv/cv100)0.2). Within this tolerance field and the relative flow positions of, i.e., cv/cv100 0.017 through 0.68 (ratio 1:40) a classical equal-percentage line can be plotted. This shows how near the ball control valve characteristic is to the equal-percentage control valve characteristic.

Fig. 4. Ball control valve inherent flow characteristic.

Fig 5 compares the inherent flow characteristics of two control valves: the well-established and accepted cage-type control valve being nominated as a valve with an equal-percentage

characteristic.

Fig. 5. Inherent flow characteristics of ball and cage-type valves.

Going back to Fig. 3 and bearing in mind that there is no way around physical laws, it can be seen that the valve is not performing as a high p reducing station in all positions. Above an 80 - 85% opening, the values for xT, FL and Z are relatively small and indicate early flashing, cavitation or chocked flow. However, this value is still large and safe enough for control loops with small or moderate p. Conversely, looking at the 10 - 75% operating positions, these values are even better or at least equal to the values of conventional control valves.

As mentioned when discussing floating vs trunion-mounted balls, no strict borderline divides linear valves and QTCVs. With the support of advanced and detailed computer programs, the advantages and limits of applying ball control valves can be easily projected. This article will demonstrate typical cases.

Flow capacity. Fig. 6 compares linear and ball control valves. It is clearly shown that the characteristic of a so-called equal-percentage cage-type control valve is not far above the characteristic of a ball control valve of the same size. In both cases, full and reduced bore, the ball control valve offers a relative flow capacity of about three times the cage-type control valve. The xT value, as mentioned, deviates strongly in the full-open position and is quite close to each basic valve type up to two-thirds of the valve opening.

Fig. 6. Six-in. ball valve vs. cage control valve.

A more detailed comparison is shown in Figs. 7 and 8. In Fig. 7, a direct size-by-size comparison is projected. As shown, ball control valves cost less for valve sizes above 4 in. It is important to mention that both valve type projections are based on standard assemblies, i.e., metal-seated valve with standard actuators. In this comparison the metal-seat ball control valve performs with tight shut-off, whereas the globe valve has a seat leakage equal to IEC 534-4 Cl.IV (0.01% of the rated cv value) as a standard.

Fig. 7. Size-by-size comparison of ball vs globe control valve.

In Fig. 8 the graphs of the two valve types are shifted so that the cv values are at approximately

projection because the cv value for a control loop is the most important scale. The graph presents facts of interest:

Actual cost of a ball control valve can be as low as one-third the price of a globe control valve with the same flow capacity

Specific cost, important for initial investment in a process plant, $/cv can be as low as 25% of the conventional control valve

Cost of the ball control valve already includes tight shut-off; the globe valve exhibits the Cl.IV leak rate.

Fig. 8. Ball vs globe control valve at equal cv levels.

A ball control valve will not be three to four times lower in price than a globe control valve in every application. The instrument engineer, planning and specifying the total package of all control valves, should be aware that there are worthwhile alternatives. And when large globe valves are needed and service conditions are too demanding for butterfly valves, it is important to consider the technology of high-performance ball control valves.

Metal-seat tightness. The quality of the valve seat performance is very important. In fact, it can be directly related to the operating cost of a process control loop. Fig. 9 shows the seat leak rates per the IEC 534-4 standard. Large control valves specified with the best seat quality, perhaps better than Cl.IV, have common problems. These are the actuator capacity holding relatively large valve plugs against high p and/or elevated temperatures that prevent use of soft seats. In most cases because of the high p and high temperature, a valve seat quality equal to IEC 534-4 Cl.IV (0.01% of the rated cv) is the best available with globe-style control valves.

Fig. 9. Valve seat leakage.

Consider the following process gas to flare system example. Such a valve should ideally be tight.

Control valve to the flare:

Size, 8 in., cv = 775, Seat quality IEC 534-4 Cl.IV

Flow coeff. of the leak rate = 0.01% x rated cv,

cv = 0.0775

Assuming: p1 = 25 barg (362 psig), p2 = atmosphere, t1 = 200C (392F), hydrocarbon gas.

Leak rate, calculated with a standard valve sizing formula, is 32 kg/h (70 lb/h) or 772 kg/day or 278 t/year. Assuming a product price of $1.20/kg, actual loss is equal to $300,000/yr.

A high-performance ball control valve metal seat will perform with a rate less than 0.001% of the Cl.IV rate.

As mentioned previously, the ball control valve seat would be made of material layers having a hardness of HRc 68 - 70 and diamond ground to zero visible leak equal to the classification of He leak detection. Most important, this technology is achievable and provides long service life.

Valve actuation cost. For medium to large control valves, actuation cost is highly important. Fig. 10 shows torque requirements and cost for metal-seated, trunion-mounted ball control valves. Depending on the nature of the fluid and/or the combination of all sliding parts, torque requirements can change significantly. As indicated, cost of small valve actuators, operating at a p value of 10 - 100 bar, is relatively small. Fig. 11 shows a direct cost comparison between actuation for globe-style control valves with a balanced trim and metal-seated ball control valves.

The valves are oriented in the actual relation, meaning not by size but flow capacity. Results show a higher actuation cost for globe-style control valves. Even a size-by-size comparison shows a much lower actuator cost for ball control valves.

Fig. 10. Torque requirements and actuation costs for ball control valves.

Fig. 11. Actuation costs for ball vs globe control valves.

Typical for a QTCV is the 0 - 90 torque characteristic. Considering the torque values for opening (break torque), controlling (running torque) and closing (seat torque), the most demanding is the break torque. In general it is more than two times the running torque. A most important feature of a quality control valve actuator is to overcome this opening mode without jumping to an uncontrolled position. The actuator introduced for such important operation works with an indirect linkage in its drive center and generates the torque increase at the position where it is needed. Such actuators are available for spring-return torques up to 24,000 lb ft (34,000 Nm) and double-acting torques up to 210,000 lb ft (290,000 Nm) [Fig. 12].

Fig. 12. Spring-return actuator.

Sound pressure level. General statements are made that QTCVs have a high pressure recovery. As described in the section on ball valve characteristics, it is obvious that such a statement has to be related to the valve opening position. Fully open, the valve may have an xT value of 0.05 or even less, resulting in chocked flow conditions at p = p1 x 0.05. At the other extreme, up to 45% opening, the xT value is >0.75, meaning equal to or better than the xT of conventional control valves. This is to say that a general statement has no validity when it comes to ball control valve parameters.

Besides the foregoing, there is the physical situation at which the energy transfer of a given mass flow and p generates a sound pressure level (SPL) above the allowable value, usually 85 dB(A). The solution to getting such a situation under control is based on influencing velocity or p within the vena contracta. The ideal solution would be pressure reduction with a constant-velocity profile, i.e., an infinite number of p steps. In addition, the mass flow or total flow capacity is divided into a number of small parallel flow portions. This, in a reasonable proportion, will reduce the SPL to an allowable value.

Fortunately the inner valve of the ball control valve generally has more space than required for the normal control mode. This ball bore is used for implementing resistors that provide multistage pressure drop. These resistors are produced from orifice plates or multiwave patterns. The multiwave pattern is based on the physical law that each 90 deviation of a flow pattern generates a pressure loss. The number of steps and/or deviations is made high enough to prevent the velocity inside the valve trim from exceeding 0.2 - 0.3 mach.

Following this technology, rated cv value of the ball control valve will decrease drastically as a result of the physical laws. As a general rule, a low-noise design will reduce the cv value by 45% or more. As shown before, the ball control valve can afford it. If the pressure reduction is not completely done inside the valve ball, additional diffusers can be made available (Fig. 13).

Fig. 13. Sound pressure level control.

Actual example. Assuming a pressure control loop for the following conditions:

Fluid

Nitrogen

Flow mn3/hr 360,000 125,000 64,000

Inlet bar 64.4 66.8 69.0

Outlet bar 60.2 54.5 52.0

Temp., C 320 320 320

MW 28.01 28.01 28.01

Isentr.exp, k 1.4 1.4 1.4

Real gas Z1 1.0182

Real gas Zn 0.9995 0.9995 0.9995

p closed 72 bar

A control valve sizing program calculates the following results:

Globe-style control valve Ball control valve

Valve size 12 in. 600# 8 in. 600#

Flow charact. Equal percentage Equal percentage

Valve flow c kvs 1,500 2,436 @90%

p ratio factor xT 0.72 0.82 0.85 0.72 0.82 0.82

Pipe red.fact. Fp 1.0; 1.0; 1.0 0.91; 0.99; 1.0

Flow coeff. kv 1,167; 250; 110 1,298; 251; 110

Outlet vel. mach 0.105; 0.040; 0.022 0.229; 0.088; 0.047

SPL dB(A) 77.7; 95.0; 95.5 75.7; 92.1; 82.1

Valve pos., % 93.6; 54.2 33.1 77.9; 37.6; 25.5

Flow pattern Noncritical Noncritical

Gain dq/ds 1.307 .847 .565 1,043, 1,089, 1,003

Fig. 14 illustrates results of a sizing computer program. Actual operating points per valve type are marked on the X-axis. Except for xT and gain, all other values are normalized to 1.0. Notice that within operating ranges of the control valves, every characteristic is perfect.

Fig. 14. Valve sizing program results.

Also, ball control valves have lower SPL values. This is the result of the two-stage pressure reduction, ball bore inlet and ball bore outlet.

The ball control valve is selected for operation between 0 and 90% opening. Actual operating points are between 25% and 78% opening. The maximum 0.23 mach outlet velocity is within the allowable value.

Of high importance is ball control valve seat tightness. The tungsten-carbide surfaces (66 - 68 HRc hardness) of the ball and seats are diamond ground. In general, the tightness results in "0 - 3 bubbles over 2 minutes" at 6 bar (87 psi) p air or, "0 - 2 drops of water over 2 minutes" with a p up to the maximum allowable value. This advantage saves product loss and it prevents erosion, abrasion or wiring due to high-velocity leak flowrates.

The following table, using calculations from a control valve sizing program, shows the cost advantage of replacing a 12 in., 600 lb globe-style control valve with an 8 in., 600 lb ball control valve.

Valve specification:

Globe-style control valve Ball control valve

Size/rating 12 in./600# 8 in./600#

Design Single seat; plug Full-bore ball

Ext. bonnet Ext. bonnet

Body A 216WCB A 105

Trim 316SS 316SS

hard-faced tungsten carbide

pressure-balanced trunion-mounted

Seat IEC 534-4 Cl. IV