Best Practices For Steam Control Values
By: Jason Carpenter
Select options that handle wide flow range and maintain tight temperature requirements.
Food and beverage applications must meet a variety of process temperature and pressure control requirements to maintain tight temperatures that help ensure product quality and safety. Selecting the right steam control system and using best practices for piping and installation can have a huge effect on production, downtime, and health and safety of end users. Implementing a rotary globe valve with a 100 to one turndown to simplify the process and allow tight performance control and shut off for the application, should be considered.
STEAM CONTROL VALVES IN FOOD AND BEVERAGE APPLICATIONS
Steam control systems, which include control valves, steam traps, and condensate recovery equipment, are critical in many food and beverage applications. Key applications include clean in place (CIP) processes and HTST (high temperature/short time) pasteurization used in dairy, cheese, milk, and ice cream manufacturing. Other widely used steam processes include retort sterilization in canning operations; bottle washers; tempered hot water systems; condensate of whey (COW) water; poultry scalders; flash steam peelers, blanchers; evaporators; direct steam injection processes; and hot air dryer systems.
These applications all use a temperature controller with a valve to maintain a separate temperature. The valve is critical, because operators must maintain very tight temperature control to avoid issues that would otherwise occur, like bacteria growth or loss of product. The challenge faced by many food and beverage applications is that they often do not have a set flow range. This variation of flow ranges is especially true for plants that run different products at different times. For example, at many plants the HTST pasteurized flow range can range from 10,000 pounds per hour to 2,000 pounds per hour. To maintain proper temperature control in the face of changes in flow, operators must use a control valve with varying rangeability and excellent shutoff characteristics.
Unfortunately, too often operators use a cookie cutter approach for steam isolation on modulating process steam equipment. For example, many systems today incorporate a pneumatically activated quarter turn ball valve to assist in shutting off the steam to the heat exchanger. They typically select the ball valve due to the standard globe valve applications in the market that have leaked. Using an actuated ball valve for isolation on the steam train to the process can lead to water hammer, pressure spikes, loss of product, or bacteria growth.
By contrast, a rotary valve has good shutoff and can meet FCI ANSI Class V shutoff on steam for these process applications. This eliminates the need for the actuated ball valves for isolation on the steam train to the process. Some rotary globe valves incorporate cam action, low friction plug operation that provides tight shutoff over long service life in a wide variety of flow control applications. Their rangeability is 100:1, allowing precise control over a wide range of flows. This enables the plant to utilize the same control valve for all the facility’s steam and fluid control applications. There can be quite an advantage to using one valve style for many applications, because it allows for plant standardization and minimal stocking requirements.
BEST PRACTICES FOR CIP PROCESS
A best practice installation for a CIP process that performs to high level expectations. The best practice setup was developed by Kevin Rasmussen, President, KEI Steam Solutions, Inc.
“The setups commonly used today typically require a lot more service and maintenance because they are not laid out or selected correctly and lack best practice piping, which can cause equipment to fail,” said Rasmussen. “Use of the suggested best practices increases productivity, reduces down time, and allows for equipment reliability on the systems. In addition to the best practice implementation shown, plant operators should always conduct an annual evaluation of steam and condensate equipment to make sure the equipment is running at optimal levels.”
CIP systems for the food and beverage industry utilize steam to heat water in the process through non-contact heating of a heat exchanger or through direct injection of the steam into the CIP tanks. The rotary globe control valve shown can be used for heat exchanger temperature control. The turndown allows for maximum temperature and pressure control across a wide range of flow variations. This is critical in CIP heating, where load variations are constant and the application requires consistent temperature control. The application requires very tight shutoff when the system is not in operation, so there is no steam and condensate losses and degradation of the heat exchanger.
Another issue in food and beverage applications is the steam trap and condensate return system, which should remove condensate quickly to allow heat transfer to occur and eliminate the stall point. The best practice solution shown includes condensate elimination for the CIP heat exchanger.
The use of a free float steam trap allows for the continuous removal of condensate from the heat transfer surface, allowing for proper heat transfer to the product. Unlike standard float and thermostatic (F&T) traps, this free-floating steam trap does not have any mechanical linkage, and has a variable orifice that will modulate with the continuous condensate load for fast efficient condensate removal and process temperature control.
In addition, the best practices set up also eliminates condensate from stalling or stacking condensate into heat transfer surfaces by utilization of a steam motor or a steam pump and trap combination. When using a modulating control valve, all condensate lines must be drained by gravity or pumped back to the boiler room. The ideal setup also includes a main pressure reducing valve for reducing the steam pressure to the CIP system from main plant pressure.
It should be noted that the best practice implementation also positions plants to take advantage of energy savings from recovering condensate, which can be like “liquid gold” to a process. This type of setup helps plant meet the required balance between energy recovery and food product performance. It helps answer the question, “How can I maintain my control without having to install costly extra bells and whistles?”
LARGE CHEESE MANUFACTURING FACILITY IN THE MIDWEST SHOWS BEST PRACTICES IN ACTION
To illustrate the benefits of a steam control system designed in accordance with the best practices discussed, here is an example of a large cheese manufacturing facility that recently overhauled its set up from an automated ball valve to the rotary valve set up.
The figure (left) shows the “before” state with a variety of improper practices:
• No drip steam trap was installed for condensate removal before the control valve to prevent valve seat wire drawing.
• The installation has an automated ball valve for isolation directly before the control valve, which offers only on/off service. This will likely cause premature failure of the control valve and heat exchanger tube bundle. It will also cause instability in the process control.
• The control valve is piped too close to the heat exchanger inlet, which does not allow for the velocities to expand out on the pressure reduction through the control valve. The minimum should be 10 pipe diameters of straight pipe run after the control valve of the heat exchanger inlet connection size.
• The heat exchanger should utilize a continuous type steam trap or variable orifice steam trap, which continuously discharges condensate to remove condensate from the heat transfer surface and allow for full latent energy utilization.
• The condensate must be gravity-drained to a condensate pump for full condensate removal.
This can be done with a steam/air motive pump or electric condensate pump.
• The setup had no thermostatic air vent for the removal of air on startup of the heat exchanger.
• Without proper air removal, the heat exchanger may experience lagging startup times, improper heat transfer, and air binding of the equipment.
• There is no Y strainer protection before the control valve to allow draining of the piping and removal of scale and debris.
The figure (left) shows the solution designed to follow the best practices outlined. A rotary control valve provides tight process temperature control and isolation in one valve. There is no need for a ball valve isolation because the rotary globe valve allows for 100:1 turndown for process control rangeability, class V shutoff, and larger CV.
Proper piping layout before and after the control valve to the heat exchanger ensures performance will be optimal. The new setup features correct use of a thermostatic air vent with vacuum breaker on the heat exchanger.
Correct discharge piping utilizing a variable orifice free float steam trap provides continuous condensate evacuation on the heat transfer surfaces. Proper drip pockets and Y strainer protects the control valve from wire drawing of valve seat. Gravity draining the steam traps to the steam / air motive condensate pump or electric condensate pump provides proper condensate removal from the heat exchanger. Expanding the piping after the steam trap, the setup includes one pipe diameter for flash allowance from the trap discharge.
IMPLEMENT BEST PRACTICES TO GET RELIABLE SERVICE AND CONTROL
Implementation of best practice equipment selection, piping, and procedures will result in reliable long-lasting service and control of equipment and processes. Unfortunately, many food and beverage plant operators do not have the technical expertise to ensure the equipment set up is properly implemented. They may be relying on vendors and process equipment providers for this service. That is why promoting best practices is critical to making the process work correctly.
With nearly 20 years of experience, Jason Carpenter is a skilled Business Program Manager at CIRCOR International. Jason manages the product portfolio of control valves, regulators, steam-fired heaters, and steam straps for steam application solutions for district energy and processing industries such as food and beverage, pharmaceutical, chemical and pulp and paper in the Americas market.