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

One such use for liquid/thermal PRDs is to protect piping and equipment from rupture due to the 

thermal expansion of liquid within an 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 released 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 material 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 already has requirements in MACT rules that require companies to follow a work practice standard 

if evidence 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. If 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 of 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 when ethylene 

glycol is released at elevated temperatures, the loss to atmosphere would create a visible 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 treatment. Any losses 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., addressing piping associated with tank 

farms or loading areas that are not co-located with a process unit, relative to the potentially 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 economic 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.