Part 1 of this series on rupture disks for Process Engineers covered why you use a rupture disk and when you might want to use this device. Part 2 discussed how to size the rupture disk. Part 3 discussed how to set the burst pressure. Part 4 discussed how temperature and backpressure affects the rupture disk specification and the relief pressure in the system. In this part, I will discuss the Relief Valve/Rupture Disk combination.

Subsequent parts will include how to specify the rupture disk and some discussion on the type of rupture disks you can purchase. Before I begin, let me point out that most of what is included in this series of articles can be found in API RP520¹ and API RP521², and ASME Section VIII, Division 1³.  Much of what is found in these documents can also be found in vendor literature.

For the relief valve/rupture disk combination (Figure 1), rupture disk sizing is totally dependent on relief valve sizing, regardless whether the rupture disk is installed upstream or downstream of the relief valve. Consequently, the discussion at this point must turn to a brief overview of relief valves.

Relief Valve Sizing Overview

Basically, the relief valve is treated as an ideal nozzle, i.e. isentropic (constant entropy) flow. A correction factor, the coefficient of discharge, is incorporated into the sizing equations to take into account the fact that this is not an ideal nozzle. The sizing equations themselves can be found in one or more of the references presented at the end.

To size a relief valve, the process engineer first determines the required relieving flow and fluid properties based on an analysis of "what can go wrong" scenarios. The flow and properties are then inserted into the appropriate sizing equation to arrive at a calculated relief valve area. If this were a stand-alone relief valve, the process engineer would use this calculated relief valve area to choose an actual relief valve from

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 Figure 1: Relief Valve/Rupture Disk Combination

a vendor catalog. But since this is a discussion of the relief valve/rupture disk combination, adjustments should be made to the calculated relief valve area before the actual relief valve is chosen.

The Rupture Disk Affect

The presence of a rupture disk acts to de-rate the relief valve capacity. This de-rating factor, called the Combination Capacity Factor (CCF), may or may not be implicitly included in the sizing formulas. Nevertheless, it is the responsibility of the process engineer to apply the factor correctly.

The Combination Capacity Factor (CCF) 

The Combination Capacity Factor (CCF) is a calculated value that is derived from data obtained during certified capacity testing of the stand-alone relief valve and the relief valve/rupture disk combination. The manufacturer first determines the capacity of the stand-alone relief valve. The rupture disk is then added, close-coupled, to the inlet of the relief valve and the capacity of the relief valve/rupture disk combination is determined. Finally, the CCF is calculated as the ratio of the relief valve/rupture disk combination capacity to the stand-alone relief valve capacity:

CCF = Flow _{Combination Capacity} / Flow _{Stand-Alone Relief Valve Capacity}

 

Below is a list of certified Combination Capacity Factors for the Continental Disc Corporation model ULTRX ^® rupture disks with the Crosby JOS/JBS Relief Valve ⁴.

Table 1: CCF's from Continental Disk

Rupture Disk Size	Burst Pressure, psig	Material	CCF
1"	60 minimum	Nickel	0.981
		Stainless Steel	0.980
3"	30 - 59	Nickel	0.981
		Stainless Steel	0.984

For comparison, the following is a list of certified Combination Capacity Factors for the Fike model MRK rupture disk with the Crosby JOS/JBS Relief Valve⁵.

Table 2: CCF's from Fike

Rupture Disk Size	Burst Pressure, psig	Material	CCF
1"	60 minimum	Nickel	0.977
		Stainless Steel	0.967
3"	35 minimum	Nickel	0.995
		Stainless Steel	0.982

Note that the CCF is a certified value and is only good for the design of the relief valve and the rupture disk that are used in the test. Since it is in the best interest of the rupture disk manufacturer to certify as many of their rupture disk designs with as many different types of relief valve designs as possible, it is typical for the rupture disk manufacturer to perform this testing and reporting of the CCF. The certified CCF will always be less than or equal to 1.0.

If the manufacturer and/or model of the rupture disk and relief valve are unknown at the time of sizing, or there is no published value for a relief valve/rupture disk combination, ASME³ requires that the CCF is not to exceed 0.9.

Applying the CCF

API Recommended Practices 520¹ shows the CCF as being applied to the denominator of the relief valve sizing equation. For example, a typical sizing equation for gas relief might look something like this:

Eq. (1)

Where:
W = required relieving rate, mass flow
T  = relieving temperature, absolute
Z  = compressibility factor
M = molecular weight
C = gas constant = a function of (C_p / C_v)
C_p = specific heat at constant pressure (consistent units)
C_v = specific heat at constant volume (consistent units)
K_d = coefficient of discharge, dimensionless
K_b = backpressure correction factor, dimensionless
P1 = relief pressure (absolute)

Note that this is the same as dividing the calculated, stand-alone relief valve area by the CCF to arrive at a required relief valve area for the combination unit:

Eq. (2)

And:

A required = A calculated / CCF

Eq. (3)

The process engineer will use this required relief valve area as the basis for choosing a relief valve from the vendor catalog.

One important thing to note is that the preceding methodology is not a requirement of code (ASME). ASME only requires that the stand-alone relief valve's certified flow capacity be de-rated by the CCF:

Flow Combination Certified Capacity = Flow Stand-Alone Relief Valve Certified Capacity x CCF

Eq. (4)

There is no mention of using the CCF to arrive at a relief valve area. Indeed, prior to the most recent edition of API RP520¹, the sizing equations themselves did not explicitly include a correction factor for the relief valve/rupture disk combination.

Note also that de-rating the certified flow capacity is only required if the rupture disk is installed upstream of the relief valve, it is not required if installed downstream of the relief valve.

Certified (Rated) Capacity

As stated above, each stand-alone relief valve will have associated with it a certified flow capacity, which is a function of both the relief valve area and the set pressure. This flow is determined by certified capacity testing procedures and is to be considered the guaranteed flow rate that can be achieved through the particular valve. With very few exceptions, this flow is used in determining both the relief valve inlet and outlet (tail pipe) line sizes. The certified flow capacity is officially stamped on the relief valve documentation. For relief valve/rupture disk combinations, the de-rated certified flow will also be stamped on the documentation.

Although the relief valve is chosen based on area, the process engineer must still ensure that the certified flow capacity is greater than or equal to the required relieving flow:

Certified Flow Capacity ³ Required Relieving Flow

If it is not, the chosen relief valve is too small. For the relief valve/rupture disk combination, the required relieving flow would be compared to the de-rated or combination certified flow capacity:

Combination Certified Flow Capacity ³ Required Relieving Flow