Understanding Basic Fundamentals of Leaking Interfaces for LDAR Monitoring – Part One

Based on data from the USA Environmental Protection Agency (USEPA) valves are the most monitored components in LDAR and Enhanced LDAR programs due to the large population of valves in applicable services. The dynamic motion of valve stems when cycled can result in frequent leaks. Although pumps have a higher leak frequency, the quantity of pumps when compared to the valve population is generally less than one percent. The quantity of connectors on the other hand is typically 3 to 5 times that of the valve population, but without moving components the leak rates are drastically less and USEPA does not require connector monitoring for most facilities in the USA. Thus, valves generally represent the largest contribution to equipment leaks on a facilities Emission Inventory (EI). This is especially true of population lacking the advancements of “Certified Low Leaking Technology” (CLLT) and a well-developed LDAR program.

Valves are the most encountered component in any LDAR program and vary greatly. Valves are used within the facility to regulate and direct flow within the process units. Designs can have a variety of interfaces that could potentially leak ranging from the bonnet/body flange to tapped auxiliary connections.

Understanding the proper terminology when talking about valves is very important. From a LDAR perspective the key terms referencing common monitoring locations are bonnet flange, packing gland/follower and stem. Other valve components include the disk and seat which are integral in regulating flow through the valve. The yoke provides the support for the handwheel, gearbox, actuator, etc. which allows an operator to cycle the valve.

Image 1 and Image 2 depict the typical design of a wedge gate valve. The chamber in which the packing or stem seal is inserted is commonly referred to as the stuffing box. The packing gland and follower are used to apply load to the top of the packing set causing it to expand radially against the stem and the bore of the stuffing box forming a seal.

Gate valves are one of the most common valves found within a facility. Gate valves are used for shut-off service allowing users to isolate equipment or direct flow as needed. Relative to LDAR programs, gate valves demonstrate a higher leak rate when compared to other valve types. This in part is due to the reciprocating

motion of the stem which can consolidate packing over time. Additionally, the stem movement can drag debris into the stem seal area when closing the valve, which can result in scratches on the stem or stuffing box creating leak pathways. Advancements in packing materials have offered a cost-effective solution to help reduce leak rates. For problematic applications, live load may be used in conjunction with CLLT packing to

further mitigate the risk of potential leaks. When monitoring a Gate valve, the following areas should be monitored per EPA Method 21 (M21) or Chinese HJ733 (HJ733).

Globe valves are designed to throttle flow rather than just operate in an open or closed position. Globe valves can have a reciprocating stem, similar to gate valves, or a reciprocating and rotating stem depending on the design. Globe valves are another high frequency leaking component, but make up a smaller percentage of valve populations than gate valves. When monitoring a Globe valve, the following areas should be monitored

per M21 or HJ733.

Gate and Globe valves are categorized as “multiturn” valves which require multiple 360o turns of the valve operator to move from fully open to fully closed. Alternatively, valves categorized as “quarter turn” only take a quarter of a turn to open and close. These are more frequently found in smaller sizes where quick opening and easy visual identification of position are needed. A ball valve is a common example of a quarter turn valve. The obturator of the valve is in the shape of a ball with a hole in it. When the valve is open the handle or lever will point parallel to the line (the hole is in line with the flow). When the valve is closed, the handle is perpendicular to the line (the solid side of the ball is in line with the flow). Ball valves have a relatively low leak frequency compared to reciprocating stem valves. Additionally, the recent publishing of API 641, users can require designs vetted via type test by changing purchase specification requirement which can reduce potential leaks. When monitoring a Ball valve, the following areas should be monitored per M21 or HJ733.

Plug valves are another type of quarter turn valve. Plug valves leak at a rate comparable to that of reciprocating valves due to features common in the various designs. Lubricated plug valves are sealed by injecting a sealant, grease or injectable packing. Plugs without an adjustable packing can have more difficulty meeting CLLT requirements. However, plug valves with and adjustable packing/stuffing box have the potential to comply with CLLT requirements via design considerations and the utilization of API 622 compliant packing materials and low emission greases. When monitoring a Plug valve, the following areas should be monitored per M21 or HJ733.

Butterfly valves are rather rare in the refining industry, but their main used mainly for throttling the flow of gas streams. The operation is similar to that of a ball valve. Butterfly valves are generally unibody, consisting of one-piece body, with a wide range of connection types: flanged, wafer, lug, etc. The common configurations, wafer and lug, are mounted between two flanges in line with gaskets sealing each side. These mounting flanges should probably be considered as connections versus a part of the valve, but some facilities classify these connections as a monitoring location similar to the bonnet flange of a gate valve. Many Butterfly valves meet CLLT requirements with the use of an API 622 compliant packing. When monitoring a Butterfly valve, the following areas should be monitored per M21 or HJ733.