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Controlling Flow With Ball Valves

Ball valves have a number of advantages over globe valves — better rangeability, turndown, higher-pressure bodies, higher capacity, 0% leakage, and usually, lower cost.

Rangeability is the useable flow through a valve, divided by the minimum useable flow.  Globe valves used in the HVAC industry typically range from 50:1 for a 6", to only 4:1 for a 1/2".  Ball valves are all about 350:1.

Turndown is the same ratio as rangeability, except the valve is installed in a system with an actuator and control signal.  Turndown ratio is the term most often used to describe a valve’s capabilities.

A globe valve with a pneumatic actuator has a turndown ratio of about 10:1.  Installing a positioner improves the turndown ratio to about 15:1.  In process control, there are special positioners available that greatly improve turndown, but they are very expensive.

Electric actuators on globe valves have a typical turndown ratio of 15:1, even on a valve with an inherent rangeability of 40:1.  These ratios will vary by manufacturer, but are typical of real world results.

When a globe valve starts to open, it will always jump open rather than move smoothly.   In order to overcome linkage and internal resistance, more movement is needed by the actuator than low flow requires.  Globe valves are installed with flow tending to push open the seat.  As soon as flow is established, the pressure pushes on the seat, the stem rises, and more flow than needed occurs.  There is a travel zone where about 10% of CV rating occurs on opening.

Ball valves do not have this jump effect.  Rotation is slow, rack and pinion linkage is used, the tendency of flow is to close a ball, and a high seal resistance, all contribute to smooth rotation without jumps, resulting in turndown ratios from 160:1 to 400:1, depending on quality of actuators, valves, and signal.  Minimum flow is fully controllable under part load conditions.  Ball valves with Belimo actuators are exceptionally good at control, and fully utilize the control signal.

Figure 1 shows ideal inherent valve characteristics.

Figure 1.

The ball valve can be described as an equal percentage valve.

Other characteristics of a typical ball valve are 400 psig ratings at 200°F for a standard bronze body valve.  Ball valves have allowable DP’s of 100 psig (check for cavitation at elevated DP’s).

Only ball valves with stainless steel balls and stems should be used.  This is especially critical in modulating valve situations. 

The problems with bronze chrome plated balls and bronze stem valves are:

   The chrome plating wears off, scratching the Teflon seats, resulting in leaks.

   Aggressive water eats away the chrome plating.

   There is a weak point between the ball and stem that will wear and create slop, and finally, total failure.

With the stainless steel ball and stem, the Teflon actually creeps into the pores of the stainless to form a good sliding surface and seal.  A ball valve with a stainless steel ball and stem should give over 20 years of service, even in modulating service.  Bronze ball and stem valves are only 5% to 10% cheaper than stainless steel ball and stem valves, and if one considers that a comparative globe valve is 30% more, there is no reason to use bronze ball and stem valves.

During the “off season”, valves have a tendency to seize up.  As an example, valves in a heating system that lays idle over the summer months.  If the valve’s actuator was sized with little or no spare torque, the valve may not work after a long idle period.  It is a good idea to supply an additional 25% torque over that required to handle additional breakaway torque.  It is also a good idea to exercise that valve periodically during the idle time.  This is easily accomplished with an actuated ball valve by using the manual handle that the valve is equipped with.  Of course, the manual handle is a convenience in an emergency situation too.  All this at no extra cost.

Most globe valve manufacturers’ literature skips over the details of what happens with valves in actual use.  They deal only with theory.  The catalog CV of a valve is supposed to be an average of three experimental results at different DP’s.  Independent testing has shown that valves with the same rated CV from different manufacturers will vary actual capacity and flow — even among the same manufacturer!  (See Figure 2.)

Figure 2.

Due to the large full-open waterway and turndown ability of the ball valve, it is much less prone to having phantom CV’s.

The characteristic curve of a ball valve will move up as DP increases due to the valve closing.  There is more flow at small stem movement than a chart will indicate.  The ball valve’s equal percentage characteristic gets close to linear flow when installed; figure 3 illustrates what happens.

Figure 3.

Figure 4.

This is a nice bonus resulting in excellent control.  The characteristic curve shape, opening against flow, is usually very different from closing against flow for the globe style valve.

This is not the case with symmetrical valves like ball valves.

The CV of a valve (in specifications) is a pipe equal in diameter to the valve with long (at least 10 pipe diameters) straight pipe runs on either side of the valve.

As was discussed in Info-Tec 13, the effective CV is less when a valve is “downsized” from the pipe size the valve will be installed in.  The amount of CV reduction is greater for ball valves and butterfly valves than for globe style valves.  This piping correction factor (FP) varies.  Generally, process valve manufacturers have FP charts available for their valves, HVAC valve manufacturers don’t.  (See Info-Tec 13 for the discussion on FP.)  Elbows near a valve change the CV and flow curves.  A minimum of 10 pipe diameters of straight pipe should be left between a valve and any other component.

In 90% of applications, a ball control valve will be a superior product in every way.  The following summary emphasizes the ball valve’s superiority over globe valves.

Apollo ball valves with Belimo actuators cost from 2/3 to 1/2 the price of globe valves with gear train or hydraulic actuators.  Installed cost is reduced, since linkage and stroking problems are avoided.

   600-psig-body rating on two-way valves, 300 psig for three-way valves

   Higher close-off pressure ratings than globe valves

   No leakage when fully shut  (Globes leak 1/2% of CV when new!)

   Turndown ratio installed as high as 250:1 with a Belimo actuator  (Average globe, 15:1.  This can be very important feature when a modulating valve is oversized, as most valves are)

   The rotation of the ball self-cleans the sealing surface

   Large CV ratings for specific pipe sizes

   Fiberglass impregnated Teflon seals are standard.

   Stainless steel ball and stem holds up under harsh conditions.  The weakest point of a ball valve is the joint between ball and stem.  Using stainless avoids this joint wearing and sloppiness, even after 20 years of modulating use.

   Belimo non-spring return actuators have a clutch release for using the manual handle provided as standard equipment.

Control of steam requires some cautions to be observed.  Actuators can be damaged by high ambients.  Special steam linkages with thermal breaks are required as supplied by Dodge Engineering Company.  The valve should be tilted so the actuator is 45 to 90 degrees from upright, so cooling with ambient airflow is accomplished.

Erosion of the ball or body is due to poor quality steam (wet steam as opposed to dry steam), and therefore DP’s should be kept to a maximum of 30 psig. even with the stainless ball and stem.  Stainless steel bodies should be considered for steam.  Strainers should be used.

In HVAC, using a ball valve to control a steam coil is an excellent choice.  When the valve first opens, the DP is high, causing the valve to have an installed linear characteristic.  The natural response of a steam coil is linear.  This results in a perfect match for excellent control.

So, how come globe valves still dominate the market when ball valves cost less and have far superior performance?  Blame both education and inertia.  Contractors and specifying engineers are just used to using globe valves.  Globe valves have been around much longer than ball valves.  Ball valves were first introduced in the process control industry in the early 1950’s and were high priced.  Now, with the recent introduction of high quality actuators such as the Belimo, and proper linkage, the ball valve package provides higher quality at lower cost than globe valves, in HVAC and process control.

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