Understanding Regulating Valves
Theoretically, flow through a particular size of open orifice area is primarily dependent upon the available pressure difference, but a real-world valve incorporates more geometry than just the orifice. While following the available passages through the valve, fluid velocity does not maintain a consistent distribution. Local pressures and momentum may be considerably different than those measured at a straight run of adjacent piping where velocities are well developed. With compressible or two-phase fluids, these effects will be accompanied by changes in the fluid density. Thus, the turns, contractions and expansions built into a valve's internal flow passages play a strong role in determining a regulating valve's maximum flow capacity and its minimum stable control capability (table 1).
Where the managed parameter is pressure, obviously, flow and pressure are interdependent. While the pressure being maintained may be at a location remote from the valve, there must always be an immediate and inherent relationship maintained among the managed flow, the pressure to which the regulator is responding and the regulator's response mode. Essentially, the regulator must be able to influence pressure at the measured point and must exercise that influence in the appropriate direction of response.
In applications where the managed pressure is upstream from the regulating valve, you normally would want the valve to open to permit flow as pressure begins to rise. Therefore, the valve's response disposition is "open on rise." The term normally applied to this mode of operation is inlet pressure regulator function. Conversely, if managed pressure is downstream from the valve, the valve's disposition would be "open on drop," and the normal reference for function is outlet pressure regulator. Specific applications call for managing a pressure differential across a regulator. In these applications, the valve normally is required to open on rise; nevertheless, the term typically used to refer to this mode of operation is differential pressure regulator function. Finally, regulators often are called upon to manage operating conditions other than pressure. Particularly where the managed parameter is temperature, the response disposition is a more definitive term.
Control TheoryFor every controlled parameter, there is a target condition, or the value the control arrangement is trying to maintain at any particular point in time. For pressure regulators, the target condition often is referred to as the setpoint pressure. For the majority of pressure regulators, target condition is constant with respect to time, but some of the more involved control schemes will adjust setpoint pressure in response to the system's specific load conditions or the requirements of the product or process under refrigeration. Such regulators are said to be compensated.
When the actual pressure being managed is different from the setpoint pressure, control theory would define the specific value of that difference as the error. An error that survives for an extended period of time indicates that the control arrangement is doing a poor job of hitting the target setting. Error is a key measure of the capability of any control device or arrangement. A control device's basic role is to maintain setpoint, which means reducing the error to zero.
In the absolute sense, eliminating error over an extended period is impossible - with expensive instruments, you always will be able to measure an error. A control arrangement that adjusts itself quickly enough that the maximum error is too low to be significant to the process being controlled represents the ultimate in cost-effective control. The error itself may still be appreciable, but it is insignificant.
When trying to establish the level of tolerable error, the most appropriate answers often are unknown in the design stages. However, the commercial realities of product offerings normally limit the reasonable solutions to a relative few. A basic feel for the process's scale and sensitivity as well as the operation will lead to proper control device or arrangement selection. Some control schemes are so heavy-handed in managing a single parameter or otherwise induce instabilities in the system they are trying to manage that the system's machinery is taxed beyond its limits and suffers in terms of longevity or integrity.
The Role of Refrigerant RegulatorsRegulating valves are used in refrigeration systems to maintain pressures or flows appropriate to either the system's operational needs or the end user's specific targets. Operational needs vary according to system complexity and duty cycle. They may include conditions such as:
- Minimum liquid line pressure required for liquid flow at the evaporator.
- Minimum oil reservoir pressure required to return or inject oil to the compressors.
- Maximum crankcase pressure required to ensure that compressor overheating or motor overload does not occur.
By contrast, end user requirements include those associated with either cooling processes or spaces down to specific temperatures, or maintaining them at specific temperatures. Each refrigeration process has its own specific targets. Product quality may be sensitive to factors such as the rate of cooling, moisture content of the surroundings and the effects of refrigeration on packaging. A second set of parameters may exist as well - for example, a minimum elevated discharge pressure may be required for a heat reclaim process.
End user requirements are targets for system performance, and close control often is the benchmark. As a result, related pressure settings and flow requirements for refrigerant regulators normally will have been established beforehand. End users want refrigeration systems to make a particular space cold, but they also realize that too cold is wasteful and may contaminate the process or service. They want the system to maintain a specific temperature, day after day. This is the key role for most evaporator pressure regulators and, in many cases, hot-gas bypass regulators as well.
Operational needs are related to system design more than performance targets. Valve arrangement selection and required regulator capacity will be specified to:
- The combination of components incorporated.
- The system's volume and extent.
- The refrigerant and lubricants selected.
- The diversity of roles to be served by the system.
- Other elements of the overall control scheme.
The designer must predict the pressure and flow requirements, select a control valve arrangement and size the control valves accordingly. That arrangement may or may not include regulators. Where regulators are employed, often their specific pressure setpoints are established at startup. A considerable range of pressure settings may have only a marginal effect on system performance.
The role for many regulators is to ensure pressure or flow conditions do not rise above or drop below certain tolerable operational limits. Realistically, that role is limited to those periods where such conditions might otherwise occur. This prevention role is a consequence of such things as low outdoor ambient temperature, startup or standby conditions, or it may be a consequence of a specific short-term system process such as a defrost cycle or lubricant transfer cycle. Some regulators spend 90% of their time in a nonfunctional role (either in a open or closed mode) and only regulate pressure for a brief interval. Other regulators are installed purely for the protection against overload or overpressure and may never modulate flow during the life of the machine.
Regulator DesignPressure regulators are manufactured for a range of applications in a number of designs. Those normally referred to as industrial valves have a flange or employ a weld-end design to allow them to be incorporated into a pipeline. Flanges can be provided to support installation in most available piping materials and permit the valves to be installed close-coupled with strainers, check valves or other components employing the same series of flange sizes. Other designs such as copper- stubbed valves simplify incorporation by accepting the pipe directly.
Most flanged valves are manufactured of ferrous materials to provide compatibility with ammonia and other industrial refrigerants. Weld-end valves are cast steel and for the most part are offered in large sizes where the greatest proportion of piping systems will be fabricated from mild steel pipe. Copper-stubbed regulators are available in iron, brass, bronze and other materials. In most cases, the incorporation method and material of construction are independent of the valve's required function or performance as a control device.
One fundamental design differentiation determines much of the selection process from a control standpoint. A regulator will be either direct- or pilot-operated. Direct-operated regulators are simple, single-minded devices intended for a specific application within a particular range of pressures. They are designed to a different scale or geometry to support a breadth of capacity and operate with permissible, appreciable error. Various industry standards, which in part serve to normalize the published data about flow capacities for such devices, require the measurement of proportionality. This characteristic represents the change in the managed pressure as flow increases. The result of those measurements is incorporated into the manufacturer's rating table for the valve.
By contrast, pilot-operated regulators are more complex and capable of serving multiple roles or functions with a single main valve design. They are designed with greater flow capacities and are able to support a much lower level of error. Proportionality rarely is measured for integrated pilot-operated valves.
Direct-Operated RegulatorIn a direct-operated pressure regulator, managed pressure is applied to some internal surface within the valve. The resulting force is transmitted to the modulating parts of the valve and balanced by a second force applied within the valve. In many cases, the second force is adjustable to establish the setpoint. Movement of the modulating parts inherently involves a change in the balanced forces and thus a change in the managed pressure. The more the demanded flow deviates from that which the regulator was experiencing at the time it was first adjusted, the more the pressure must deviate from setpoint.
Direct-operated pressure regulators frequently are employed on compact systems, partially because as the flow area increases, the internal parts and forces being managed get quite large. They often are applied where refrigerant flow is not expected to vary over a wide range and where error is not particularly critical. For systems having only one regulated evaporator circuit or one compressor, refrigerant flow itself does not greatly vary and pressure control does not have to be precise.
However, for an evaporator pressure regulator, this characteristic causes the inlet pressure to rise along with the refrigerant flow requirement. The demand for in-creased refrigeration is accompanied by a rise in evaporating temperature, an obvious dichotomy.
The decision to select a direct-operated regulator for a multicompressor, multievaporator central system is questionable. The flow for a given circuit can vary considerably from one point in time to the next, and flow can change drastically according to an operational mode such as hot-gas defrosting or heat reclaim. In many processes, precise pressure control is needed even though the system itself is limited in capacity.
Pilot-Operated RegulatorA pilot-operated regulator consists of a pilot section and a slave section. Two flows are managed by the valve: The first, which is by far the greater proportion of the total flow, is managed by the plug and orifice in the slave section. The second, the pilot stream, is managed by the pilot section. The pilot section may have its inlet connected to the immediate upstream side of the main valve or may source its flow from a higher pressure space. The pilot section's modulating portion is available in all dispositions and functions of direct-operating regulators. Versions that manage pressure employ some elements of direct-operated regulators in their throttling elements, but the slave section captures the resulting managed flow in a nearly trapped space. The most common designs use a piston to press against a plug in the opening direction.
The slave element often is called the main valve and incorporates a relatively low force closing spring. Because the piston is considerably larger than the plug and because the spring requires only a minor force to compress it, a small change in top-of-piston pressure readily moves the piston and plug and causes a large change in the slave section's available flow area. The pilot section manages limited flow through a bleed hole and piston-bore clearance, so a small change in pilot stream flow creates a considerable change in the top-of-piston pressure. Thus, a small movement of the pilot section is all that is required to fully open the main valve.
In the integrated version of the valve, once again, two parallel flows are managed. The first, which is by far the greater proportion of the total flow, is managed by the plug and orifice in the slave section. The second, the pilot stream, is sourced from the valve's inlet side and eventually exhausted past the piston to the valve's outlet side. Carefully designed, such an integrated valve requires a pressure difference less than 2 psid to overcome the closing spring forces and will reach 100% of its effective stroke with a pressure change of only 0.4 psi. When the flow required to maintain the setpoint decreases to a fraction of the valve's maximum capacity at the prevailing pressure difference, the valve's portion will become unstable and hunt. At low pressure differences, this hunt will be slow and involve a relatively small error, but with large pressure differences or inherently unstable flows such as those that accompany the expansion of warm, high pressure liquids, the hunt can get quite violent and challenge the longevity of the valve's internals.
The main valve section of a pilot-operated regulator often is derived from a piston-activated, pilot-operated solenoid valve. The pilot section for such a valve is simply a smaller solenoid valve. This setup allows users to utilize both the pilot solenoid and one or more of the pressure pilots on the same valve. The solenoids are used in an override sense to alternately effect closing or opening of the main valve. In other words, an inlet pressure regulator with electric shutoff employs a solenoid pilot to interrupt the pilot stream when the solenoid is closed, thus causing the main valve to close. When the solenoid pilot opens, the valve reverts to its automatic mode but may remain closed.
The solenoid pilot effects a "when" decision - the decision itself being made by an element of an electrical control scheme - and its only alternatives are open or closed. With the pilot solenoid open, the valve is permitted to function as a regulator. The main valve modulates to maintain inlet pressure at the setpoint, which means it may be in any position from fully closed to fully open in response to that pressure condition.
The pilot solenoid also may be used to select between two different pressure pilots where the managed pressure has one desired setpoint for an established time interval and a second desired setpoint at a different time interval. Where the regulator function is the same for both pressure pilots, the result is a dual-pressure regulator. For more complex cases, solenoids can select from one pilot disposition to another. For a select period, an integrated valve could function as an inlet-pressure regulator, and at other periods, it could function as a differential-pressure regulator. The main valve no longer has a single functional description.
Valves also can be provided to modulate in response to controlled parameters besides pressure. Temperature regulators are available in dispositions that include open on rise and open on drop. Compensated-pressure pilots can be added to adjust setpoint automatically via a pneumatic signal or electromechanically by a damper-style motor. Or, a main valve can be piloted via an electronic controller and positional solenoid so all the permutations of open, close, multidisposition and so on can be driven by external electronics. The permutations that can be derived from these pilots are almost limitless - well beyond what would have practical application.
Many of these devices are used in food and process industries, and the regulator applications are as diverse as the refrigeration plants themselves. The refrigeration requirements represent large loads with installation of high horsepower motors, and the evaporator installations and piping systems are sprawling. Applying manpower to making adjustments to the refrigeration plant is not reasonable at these levels of complexity and remoteness. While energy costs have not been escalating as they once did, the costs of measurement and monitoring and remote communications have decreased, so control is a more readily effected measure than it was 10 or even two years ago.
In the future, control valves will be designed not as free-standing devices but as one element of these expansive, integrated control schemes. As the electronic control industry rapidly consolidates and the various protocol, transmission, processor frame, algorithm and other diversities normalize, the performance requirements for regulators will become more detailed and difficult to address. Future regulator designs will not only need to fit within the expanded control sphere but will have more detailed performance requirements than they do at present.