V. Prabhu, Worley Services India Pte. Ltd., Navi Mumbai, India
Clause (g) of UG-156 of the American Society of Mechanical Engineers (ASME) “Boiler and pressure vessel code,” Section VIII, Div. 1,1 allows the use of stop valves in the pressure safety valve (PSV) relief path, provided that the stop valves are constructed and positively controlled so that the closure of the maximum number of stop valves possible at one time will not reduce the pressure relieving capacity of the unaffected pressure relief devices below the required relieving capacity.
Clause 8.3.2 e) of the American Petroleum Institute (API) Standard 520 Part 2 suggests the use of a suitable interlocking system to prevent the accidental closure of isolation valve(s) of PSVs in service before opening the isolation valve(s) of a PSV installed as a spare.2
The language used for writing mandatory codes and standards is often difficult to understand, making it even more difficult to visualize the sequence in which the interlocks work.
This article will explore the most commonly used devices for changing over the PSV from a service PSV to a spare PSV, or vice versa. The objective is commonly achieved by two methods:
Change-over valve
Mechanically interlocking PSV isolation valves.
The basic purpose of an interlocked or change-over valve(s) is to ensure an open path for relief when switching over to the spare PSV. This ensures that at any given point, the required relief capacity is available and the safety of the system is not jeopardized in any way.
Both methods have their own merits and demerits/limitations.
A change-over valve, sometimes called a safety selector valve (SSV), is designed specifically as an effective ‘switchover’ device that permits routine or emergency servicing of redundant pressure relief devices with no process interruption, providing continuous system overpressure protection. FIG. 1 illustrates a typical change-over valve.
Most commonly, a change-over valve is installed at the inlet of the two PSVs. FIG. 2 shows a change-over valve installed at the PSV inlet.
It is mandatory to limit pressure drop in the PSV inlet piping to 3% of the PSV set pressure, so it is vital to ensure minimum pressure drop in the change-over valve. Published literature by change-over valve manufacturers provides CV values (KV in metric) according to the model number and size of the valve. The higher the CV or KV value for a given flow, the higher the valve size and lower the pressure drop across the valve.
If the fluid to be relieved is toxic and/or flammable, a change-over valve is installed at the PSV outlet, as well. FIG. 3 shows change-over valves installed at a PSV inlet and outlet. This combination requires only one set of piping at the inlet and the outlet. Handwheels of inlet and outlet change-over valves are connected via a chain to ensure synchronized opening and closing. A lock is provided to prevent unauthorized operation.
One limitation of change-over valves is that, in a multiple PSV installation, each PSV should have one change-over valve. This increases installation costs.
Mechanical interlocks are cheaper and more reliable than change-over valves. They eliminate human error by allowing only selected valves to be opened or closed in the correct sequence. Mechanical interlocks offer another advantage.
An interlock is installed on the original isolation valve without modifying it. Its original handwheel or lever is removed and replaced by an interlock with a new lever or handwheel. A typical interlock assembly with a handwheel is shown in FIG. 4.
A typical sliding key is shown in FIG. 5. The sliding key is inserted into a slot that requires only linear movement, making the design robust and minimizing wear and tear. A periodic inspection is all that is needed to maintain the interlock is in good condition.
In the following sections, PSV change-over is explained step-by-step with the position of the PSV and its isolation valves after each step.
FIG. 6 depicts a situation where PSV A is a duty valve and PSV B is an installed spare. Both PSV outlets are connected to a flare system and have isolation valves installed on their inlet and outlet piping. The isolation valves are full-bore and installed with their step in horizontal position as per API 520 guidelines. PSVs are mechanically interlocked such that the inlet/outlet valves of the duty PSV are locked open. The inlet and outlet valves of the spare PSV are locked close and locked open, respectively.
There are five unique sliding keys (Keys A, B, C, D and E) associated with the four manual isolation valves. Each valve has an interlock assembly installed on it with two slots for entering designated keys.
Each isolation valve has one key in its slot. The fifth key (Key B) is in the key control cabinet inside the control room.
If PSV A is to be taken out of service for maintenance work, then PSV B should be brought into service first. The following steps should be taken:
Step 1: Remove Key B from the control cabinet and bring the key to the field. Insert Key B into the slot of the interlock assembly of the PSV B inlet isolation valve. Turn the handwheel to open the valve. After the valve is opened completely, Key C is released from its slot. Refer to FIG. 7 for the valve position after Step 1 has been completed. Note:
The movement of the handwheel (or lever) is possible only when both keys are inserted.
The second key is released only after the handwheel rotation is completed.
Step 2: Insert Key C (released in Step 1) into the slot of the interlock assembly of the PSV A inlet isolation valve. Turn the handwheel to close the valve. After the valve is closed completely, Key D is released from its slot. Refer to FIG. 8 for the valve position after Step 2 has been completed.
Step 3: Insert Key D (released in Step 2) into the slot of the interlock assembly of the PSV A outlet isolation valve. Turn the handwheel to close the valve. After the valve is closed completely, Key E is released from its slot and then returned to the key control cabinet in the control room. Refer to FIG. 9 for the valve position after Step 3 has been completed (FIG. 10).
Step 4: Key E is returned to the key control cabinet in the control room.
After depressurization by opening the vent and drains valves, PSV A can be removed and taken out for maintenance (FIG. 11).
Takeaway. From the discussion it is understood that mechanically linking PSV isolation valves offers a better solution compared to change-over valves. The removed key is kept inside a cabinet in the control room. Standard operating procedures should ensure that only authorized personnel have access to the key. HP
LITERATURE CITED
American Society of Mechanical Engineers (ASME), “Boiler and pressure vessel code,” Section VIII, Div. 1, 2023, online: https://dl.gasplus.ir/standard-ha/Standard-ASME/ASME%20BPVC%202023%20Section%20VIII%20div.%201.pdf
American Petroleum Institute (API) Standard 520, Part II, “Sizing, selection and installation of pressure-relieving devices in refineries—Installation,” 7th Ed., October 2020.
Leser, “Leser change-over valve catalogue,” September 2020, online: https://www.leser.com/en-us/products/best-availability-change-over-valves/
Haake Technik, “Technical bulletin of HSV® interlocking systems,” online: https://www.haake-technik.com/en/products/valve-locking-systems-hsv/
VINAYAK PRABHU is a Deputy General Manager Process at Worley Services India Pte. Ltd. in Navi Mumbai, India. He has more than 29 yr of work experience as a process engineer in engineering, procurement and construction (EPC) companies. He earned a B.Tech degree in chemical engineering from Nagpur University, India, and is a Chartered Engineer (CEng MIChemE) in the UK. The author can be reached at Vinayak.Prabhu@worley.com.