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Volume 32, Issue 5
Valve design and materials affect performance and cost to maintain.
The pharmaceutical industry is under significant pressure to optimize production and control manufacturing and maintenance costs. When assessing the costs to build, operate, and maintain a production system, it is crucial to understand how each component in the process contributes to total cost and system reliability. While hygienic diaphragm valves are a major contributor to costs, best practices and new technologies can be implemented to reduce these costs and improve performance.
Due to the complex nature of the manufacturing processes used in bioprocessing and the high degree of sterility required, the industry uses hygienic diaphragm valves where stainless-steel systems are employed. A stainless-steel-based bioprocessing plant typically has thousands of diaphragm valves across the entire production process. Processes such as water treatment, media and buffer preparation, harvest, and purification all employ diaphragm valves extensively.
Diaphragm valves are considered the cleanest and most drainable valve solution for these applications. Because the industry also requires the majority of its systems to be cleaned in place (CIP) and steamed in place (SIP), valves must deal with a wide range of temperatures, pressures, chemicals, and compounds and meet required material properties for components in contact with drug product.
Figure 1. Conventional diaphragm valve
[All figures are courtesy of the author].The number of diaphragm valves utilized in these systems makes them one of the most critical components in the drug manufacturing process. It also means that valves are one of the largest maintenance costs in time, material, and labor. By design, the valve is robust, but proper maintenance intervals and techniques are required to retain optimal performance, process reliability, and system integrity.
To optimize the use of diaphragm valves in bioprocessing, manufacturers must understand the valve design and factors that affect valve performance and reliability.
Conventional diaphragm valves (see Figure 1) are constructed with a valve body, diaphragm, top works, and four bolts. The bolts clamp the valve body, diaphragm, and top works together. The pressure boundary formed by the diaphragm and valve body joint is critical to maintaining the sterility of the process.
There are many factors that can impact seal reliability. However, some of the most common factors seen in the industry today are cold flow and compression set, damaged bolts, and point loading.
Cold flow and compression set
Most bioprocessing applications use polytetrafluoroethylene (PTFE) diaphragms because they are resistant to steam and chemically inert. PTFE also resists the common media, pressures, and temperatures used in bioprocessing. However, during thermal cycling the PTFE performance is challenged by the hot and cold temperature extremes. The PTFE material lacks the rebound properties of elastomer materials and will cold flow (thin) when subjected to changes in pressure or temperature.
A thick elastomer backing cushion is utilized with the PTFE diaphragm to maintain seal integrity during thermal expansion and contraction. This backing cushion effectively becomes a spring in the joint, which maintains a sealing force. As the valve goes through thermal cycles, the backing cushion is compressed during heating and expands during cooling. The elastomer does not fully recover each time, but takes a permanent compression set, which deforms the diaphragm. This thinning of material will degrade the valve pressure capability over time, eventually leading to leaks and risk for contamination.
It is critical for the end user to understand their process and determine what duration of exposure and amount of thermal cycles can be attained before cold flow or compression set causes a leak. Based on this, the manufacturer should determine a schedule for replacing diaphragms.
To maximize the thermal cycle performance, proper torque and re-torque practices are essential. PTFE diaphragms must be torqued gradually, increasing torque from hand tight to the manufacturer’s specification in a multi-pass technique (see Figure 2).
Additionally, after an initial thermal cycle, the conventional four bolt valve should be re-torqued in the same method, typically once the system has cooled to normal temperature.
Seal integrity is paramount to ensure sterility, but the seal reliability is dramatically affected by the simplest valve components: the four bolts that clamp the valve components.
In general, the bioprocessing industry is averse to using lubrication near the production process. Therefore, bolts are typically not lubricated. However, using unlubricated bolts leads to rapid galling when the bolts and nuts are tightened. This galling occurs after only one use of the bolts. After five uses, the galling is so significant that the clamping force of the valve assembly can by reduced by as much as 20% (see Figure 3).
Figure 3. Clamping force reduction after five installations.
As a result, seal integrity is substantially reduced and will vary widely depending upon the severity of damage to the bolts. Due to the forces required to achieve a seal with PTFE diaphragms, even one bolt that is seized or not properly tightened can lead to risk of a process leak and contamination. Therefore, it is critical to valve performance reliability that bolts be routinely assessed and replaced to ensure optimal and consistent valve performance. Alternatively, valve designs with no bolts can be used to eliminate galling, torqueing, and re-torqueing issues and improve the valve’s reliability (see Figure 4).
Figure 4. EnviZion (ITT Engineered Valves) valve design with stud and bonnet keyway slot instead of bolts.
Another common seal integrity performance issue is seat sealing, which is created when the valve is closed to stop flow. As the valve closes, the top works forces the diaphragm against the valve body weir. The performance of this seal is significantly affected by how the valve diaphragm is installed during the preventative maintenance process. If the diaphragm is installed incorrectly, then it will become damaged and not seal properly on the weir. PTFE diaphragms can be damaged with a condition commonly called point loading (see Figure 5).
Once the diaphragm is damaged via point loading, it must be replaced. If it is not replaced, the seal integrity of the valve is in jeopardy, potentially leading to process contamination.
Having knowledge of these important factors is critical in developing a comprehensive preventative maintenance program to prevent these factors from causing process leaks and risk for contamination. Working within the constraints of the valve design and performance will result in a reliable process.
A European biopharmaceutical manufacturer experienced a number of valve issues on a critical process line. The line utilized sodium hydroxide as a cleaning agent for the CIP system. Several diaphragm valves on the line were leaking within three months after diaphragm replacement and repeated bolt re-torques. Leakage of the sodium hydroxide solution into the interstitial space created alkaline deposits on the valves and caused a health and safety concern. While drip trays were installed to provide a short-term fix, a full evaluation and permanent solution to the leaking valves was required as part of the manufacturer’s good manufacturing practice (GMP).
The line was shared by multiple processes, making it difficult to schedule maintenance time to repair the valves. During a routine maintenance turnaround, the valves were inspected and symptoms of seal failure were identified: over-compressed diaphragms due to multiple re-torques and damaged bodies due to caustic residue. Having operated in these conditions for many years, the valves no longer maintained a tight, repetitive seal to atmosphere and needed to be addressed.
The manufacturer decided to replace the valves and sought a new valve assembly that would meet specific performance criteria: improve maintenance reliability and repeatability, decrease maintenance time, and reduce external leaks. Additional criteria for the new valves included matching the current valve flow rates, fitting into the existing piping space, and functional equivalency.
A custom block valve was designed for the manufacturer (see Figure 6). The valves were specified to match the existing flow characteristics, which ensured that the process and cleaning system remained unaltered. The valve type (EnviZion, ITT Engineered Valves) uses an active thermal compensation system that provides a constant sealing force against the valve body and diaphragm over varying operating. The valve uses a patented quick connect stud design that removes variability in the installation process and assures the diaphragm is installed correctly.
By troubleshooting, following best practices, and implementing new technology, manufacturers can overcome the industry’s most challenging applications and keep production on-line with fewer interruptions. Installing, operating, and maintaining valves more efficiently will increase productivity, improve reliability and cleanability, and reduce operating costs.
Vol. 32, No. 5
When referring to this article, please cite it as P. McClune, “Optimizing Diaphragm Valves to Improve Bioprocess Reliability," BioPharm International 32 (5) 2019.
Paul McClune is global product manager, ITT Engineered Valves.