Working with valve arrays for fluid transfer has always been an issue, involving complex panels of valves and safety problems due to errors and poor labeling. Mixproof valves can simplify panels, increase safety and ensure product integrity.
Mixproof valves each allow two isolated flows, or mix two flows together. “In general, they’re safer than using conventional valves, and more flexible as they can control flows from any source to any destination,” said Andres Kao, process automation engineer, Emerson, in a presentation at the 2019 Emerson Global User Exchange in Nashville, Tennessee.
Kao said, “With mixproof valve arrays replacing the conventional transfer panels, we will need some implementation ideas on how to make its DeltaV design compatible with the PCSD,” the Project Management Office Configuration Standard. His presentation focused on mixproof valves and their valve states, the logic behind the mixproof valve array control module, and how a mixproof valve array equipment module is designed to match PCSD standards as much as possible.
How mixproof valves work
Each mixproof valve has a top path and a bottom path. Their states are: both closed; mix top and bottom; upper lifting (drains top); and lower lifting (drains bottom). They are equipped with multiple limit switches to verify their state.
In a typical flow control panel, the valves are used in an array of rows and columns. One source connects to all the top paths, and one destination connects to all the bottom paths.
“There is no standard library in DeltaV for a mixproof valve or array,” Kao said. “There is also not an independent control module—each valve has two possible flows so they can have more than one ownership and not just one owner ID.” The valves can be used by multiple phases, two units and/or two products. Further, the status of one valve in an array depends on the status of the array as a whole, and an array can be used by multiple operations.
“We need to deal with mixproof valves in a way that’s consistent with the rest of the project to meet PCSD standards,” Kao said.
For example, an array of three rows and five columns would handle three sources (one per row) and five destinations (one per column). A source is connected in series to the top path of all the valves in its row. A destination is connected in series to all the bottom paths in its column.
In its DeltaV representation, a source will own all the top paths in its row. A destination will own all the bottom paths in its column. A flow is defined by its source and its destination. A source equipment module (EM) may always own its top path, and a destination EM may always own its bottom path.
To prevent cross-contamination, no two (or more) flows may share a common source or a common destination, therefore, do not open (mix) a mixproof valve unless both the top path and the bottom path are used for the same flow. However, the same array can simultaneously feed two separate destinations from two separate sources.
As a result, for N sources there will be N source EMs that own the common source (top path). These EMs have a destination assignment that determine their ownership of the destination path (bottom path). A manager control module (CM) is needed to ensure no flows may share a common destination or a common source.
Definitions for DeltaV
The definition of a mixproof valve is: Valve contains a top path, a bottom path, and a drain path. It allows paths that are separated by a device that: Opens (mixes); Upper Lifts (top-drain); Lower Lifts (bottom-drain); Closes (isolates).
The DeltaV mixproof valve CM has a top path Owner and a bottom path Owner. The DeltaV mixproof valve CM will have the following EDC states:
The DeltaV mixproof valve CM has two additional commands:
“Remember to have enough limit switches to know if your valves are in the status they’re supposed to be in to avoid mixing,” cautioned Kao.
For a given EM with source Y and destination X, if the EM is owned, it will acquire the appropriate valve tops and bottoms to complete the needed path using the appropriate CMs. For example, CMij is the mixproof valve CM for source i and destination j.
The rule for the EM end command for an open mixproof valve (top is owned and bottom is owned) is “close when done.” “Remember that the top path is always owned by the source EM and the bottom by the destination EM.”
For an upper lift mixproof valve (top is owned, bottom is not), the rule for the end command is “only if top is owned.” Similarly, for a bottom lift (bottom is owned, top is not), the rule is “only if bottom is owned.”
The sequence is:
1. A phase acquires the source EM via an alias and sets its owner ID and destination.
2. The mixproof array EM sends its destination to the CM manager.
3. The CM manager approves (or disapproves) given the source EM and its destination:
• No two or more flows may share a common destination or a common source
• No two or more EMs may execute their logic at the same time (queue)
4. The phase may fail if the CM manager disapproves. Otherwise, it will continue executing its logic.
Advantages and an idea
“The benefits of this approach are that you can generally reuse the PCSD logic with minimal change at higher-level entities,” Kao said. The acquisition can be edited to account for the assigned destination, and a source EM alias can be used to specify the source unit. It also requires minimal graphic changes, as you can reuse most of the dynamo functions (CM), reuse the faceplate (CM), and reuse the faceplate and dynamo (EM).
“However, since no two or more flows may share a common source or destination, clean-in-place (CIP) flows can’t go to multiple sources at the same time. And, since a source is always owned by an EM, you can’t have multiple sources for a flow,” Kao said.
A potential improvement would be to use one EM for the array. “The manager would rest in the EM, and each path would be truly defined by sources and destinations, not constrained by the source unit or destination unit used,” Kao said. “It would involve fewer modules (and points of failure), but might require some uncommon practices to handle.”
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