There are several different uses for PRDs on liquid lines at chemical facilities.

One such use for l iquid/t hermal PRDs is to protect piping and equipment from rupture due to the thermal expansion of liquid within a n isolated section of piping/equipment ( blocked

- in by closing valves on both ends)

. The risk of over pressure does not come from process or pipeline operating conditions, but instead from the increasing pressure of the contained liquid as its temperature increases due to atmospheric conditions. The amount of liquid that must be release d to prevent over pressure (pipe/equipment rupture) is very small, and the duration of such releases is very short

. These thermal expansion relief valves typically must discharge only a few fluid ounces of liquid to bring the equipment back into a safe pressure range. This discharge is easily detected during daily visual inspections when the materi al has an odor, is colored , or when a device does not fully re

- seat
. In some cases, discharges from a liquid PRD are routed to a process vessel , collection vessel, or a control system, but may also be collected in a secondary containment area and routed to a wastewater treatment system. The current proposed PRD regulatory language already excludes PRDs that release to a process vessel or a control system, and a similar exclusion should be added for sewer systems. Discharges from a liquid PRD can contain heavy liquids with essentially no emissions. Examples of heavy liquid HAPs include ethylene glycol and a number of glycol ether compounds. Light liquids that are discharged may or may not result in air emissions depending on where they are discharged and how they are managed after the discharge. EPA alread y has requirements in MACT rules that require companies to follow a work practice standard if ev idence of a discharge from a PRD in liquid service is present based on an AVO inspection

. One reference is Section 63.169 of the HON Rule, which is below

. The text in § 63.1010 (TT) and § 63.1029 (UU) is very similar.
**§63.169 Standards: Pumps,**  **valves, connectors, and agitators in heavy liquid service; instrumentation systems; and pressure relief devices in liquid service.**  (a) Pumps, valves, connectors, and agitators in heavy liquid service, pressure relief devices in light liquid or heavy liquid service , and instrumentation systems shall be monitored within 5 calendar days by the method specified in §63.180(b) of this subpart if evidence of a potential leak to the atmosphere is found by visual, audible, olfactory, or any other detection method. I f such a potential leak is repaired as required in paragraphs (c) and (d) of this section, it is not necessary to monitor the system for leaks by the method specified in §63.180(b) of this subpart. (b) If an instrument reading of 10,000 parts per million or greater for agitators, 5,000 parts per million or greater for pumps handling polymerizing monomers, 2,000 parts per million or greater for all other pumps (including pumps in food/medical service), or 500 parts per million or greater for valves, connectors, instrumentation systems, and pressure relief devices is measured, a leak is detected. (c)(1) When a leak is detected, it shall be repaired as soon as practicable, but not later than 15 calendar days after it is detected, except as provided in §63.171 o f this subpart.(2) The first attempt at repair shall be made no later than 5 calendar days after each leak is

detected. (3) For equipment identified in paragraph (a) of this section that is not monitored by the method specified in §63.180(b), repaired shall mean that the visual, audible, olfactory, or other indications of a leak to the atmosphere have been eliminated; that no bubbles are observed at potential leak sites during a leak check using soap solution; or that the system will hold a test pressure. (d) First attempts at repair include, but are not limited to, the practices described under §§63.163(c)(2) and 63.168(g) of this subpart, for pumps and valves, respectively.
There are a variety of ways that leak detection occurs for liquid service PRDs. Those routed to collection vessels would trigger a liquid level alarm if sufficient material were released into the collection vessel. Those routed to sewers may be observed via the normal monitoring of wastewaters such as finding abnormal concentrations of COD. Those routed to containment areas would be observed visually. An ethylene glycol drip or release is typically easy to detect by its nature. Even whe n ethylene glycol is released at elevated temperatures, the loss to atmosphere would create a vis ible fog or mist as it quickly condenses in ambient air. Typically, ethylene glycol losses from liquid PRDs are collected, captured, routed back to the process, or routed for disposal either via management as a solid waste stream or via biological treatmen t. Any l osses to atmosphere are believed to be exceptionally small. ACC member companies believe that their existing processes for monitoring (i.e., AVO inspections of piping, wastewater composition analysis) timely reveal leaks and spills, and these leaks are being reported under a variety of existing applicable requirements. Finally, implementing the individual PRD monitoring or parameter monitoring requirements for PRDs in liquid service could prove both challenging and expensive, e.g., add ressing piping associated with tank farms or loading areas that are not co

- located with a process unit, relative to the potential ly small air emissions involved in some cases

. One ACC member company estimated that it has over 100 applicable PRDs in liquid service subject to P&R IV, with the vast majority of these primarily in ethylene glycol service or combinations of ethylene glycol and monomers. The costs of requiring monitoring for these PRDs in liquid service have not been accurately captured by the current e conomic analysis. For this and the other reasons noted above, we believe that EPA should only require PRDs in organic HAP gas/vapor service to comply with the new and expanded PRD requirements

.