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Monday, January 5, 2009

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In recent Hydrocarbon Processing November 2008 release, an article "What You should know about liquid thermal expansion" has discussed about issue related relief due to liquid thermal expansion. There was similar discussions in previous posts such as :
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Similar to what has been presented in "Simple Flow Chart to Determine Requirement of Thermal Relief", this article presented a Decision Flowchart assist in determining requirement of Thermal Expansion Relief Valve (TERV or TRV) in detail and brief in TRV relief rate, location of TRV and discharge point. Certain points discussed in this article is a bit brief and quickly jumped into the conclusion. Thus, the intention of this post is to further expand it, provide some highlights and pre-caution so that reader and user can take serious considerations on decision of provision of TRV.

Definition of Liquid Filled System & Cushion effect
R1 : No TRV is required if it is not completely liquid-filled system. A system is considered completely liquid-filled at 95% or greater volume. For a two-phase system with lower liquid fraction and gas system, a TRV is not ussually required because existing gas can absorb the increase in liquid volume due to thermal expansion.
Consideration if gas "cushion" effect for liquid system is acceptable and TRV may not be required, one shall take note that the "cushion" effect may not be applicable to condensing vapor and/or vapor is highly soluble in liquid. As heat input (external or internal) to the system, liquid expands and pressure increase. Vapor may condense to form liquid or vapor may dissolve in liquid rapidly. This quickly removed the "cushion" effect readily available in vapor and results pressure rise rapidly.

The recemmendation of not having TRV in 95% liquid system may be only applicable to some conditions but not all conditions. It is very much subject to liquid expansion factor. Liquid with high expansion factor i.e. light HC would tends to remove the vapor "cushion" effect quickly. It is always recommended to study the possibility of overpressure due to thermal expansion using constant density method. Besides, for close system, it has some level of difficulties to ensure or guarantee of less than 95% liquid volume requirement. Thus, this rule shall be applied carefully.

Non-Blocked in Section Shall be Drained... Losses
R2 : Process plant piping (on-plot piping) is not normally blocked in, but storage or transport piping sections are regularly shut-in during normal operation. This is why few process piping lines have TRVs.
This recommendation is valid for process piping is not normally blocked in during normal operation. When it is blocked, part of liquid is possible to be drained (sufficient vapor space) to avoid overpressure due thermal expansion. Quantity of liquid to be drain to provide "sufficient" vapor is arbitrary and shall be stuided when this philosophy is adopted. Provision of 10% vapor space could be a good start.

Generally process piping within a plant is not equipped with TRV based on above consideration. However, some operating owner may not agree to lose any inventory i.e. long distance pipeline, due to draining activities. Thus, TRV may still be required. If above philosophy is adopted, the draining requirement after blocked in shall be agreed and captured in the operating procedure. Used of Locked-Open (LO) manual isolation valve couple to strict permitting procedure is one of the good practice to minimize potential of complete blocked-in.

TRV Not Required for "Piping" ?
R3 : The heat source can be internal or external. An internal heat source is.... External fire is not considered a heat source if thermal expansion is studied for a system consisting of only pipes...
Above could be a consequence of code mis-interpretation where pressure containing part designed to PIPING code i.e. ASME B31.3, there is waiver from overpressure protection. For example, a DN1500 pipe and a DN 150 filter containing same fluid, same design pressure and temperature, the volume contains by the DN1500 pipe significantly higher than DN150 filter. As per code interpretation, PSV is provided for DN150 filter but not required for DN1500 pipe. This consideration is unreasonable as the risk for DN1500 pipe is much higher than DN150 filter.

Requirement of PSV shall NOT be 100% judged from the design code itself. The RISK and CONSEQUENCE e.g. INVENTORY associates risk & consequences shall come into consideration to define if a overpressure protection i.e PSV is required. This has been discussed in "Requirement of overpressure protection devices on system design to PIPING code".

Concluding remark : TRV is Just a Small & Simple Overpressure Protecting Device
TRV is a small, simple and non-costly overpressure device. Compare to other major equipment such as compressor, turbine, column, etc, it is just almost nothing to compare from cost aspect A TRV can be a DN20 x DN25 PSV. it is nothing to compare from the associated consequences for not providing it. Seriously analyse it before decision on not providing it for thermal expansion relief.

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posted by Webworm, 1:35 AM

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