Cost Comparison Between Comparable Stand-Alone Rupture Disk and Relief Valve

Rupture disk manufacturers burst at least two disks per lot before shipping them to a customer.

As a consequence even if you want just one rupture disk you will be buying three. Therefore, the first usable rupture disk is comparatively expensive. Also for new installations, each installed rupture disk must be purchased along with a holder. However, the same holder may be used for replacement purchases as long as you buy the exact same rupture disk from the same manufacturer.

Below is a capital cost comparison between Continental Disc Corp. (www.contdisc.com) 3" Ultrx Hastelloy C rupture disks with holders and Farris Engineering (www.cwfc.com) 2600 series relief valves, based on a budget estimate in year 2001 dollars.

 

Table 1: Cost Comparison - Rupture Disk vs. Relief Valve

Basis: Continental Disc	Basis: Farris Engineering
3" Ultrx Hast C Disc = $2,600 for 1st usable disk, then $870 each	3" x 4" Hast C 26KA10-120 = $13,400
3" Ultrx Hast C Holder = $3,300 ea.	Â
TOTAL for one pair = $5,900	Â
TOTAL for three pair = $14,240	TOTAL for three = $40,200

This capital cost comparison will vary considerably with size and material of construction but you get the point. However please note that everything has a value and the loss of contents should be considered in the overall cost difference between a rupture disk and a relief valve.

When Do We Use a Rupture Disk-Relief Valve Combination?

Rupture disks are often used in combination with and installed just upstream and/or just downstream of a relief valve.  You may want to choose the combination option if:

Figure 3: Rupture Disk- Relief Valve Combination

 

1.  You need to ensure a positive seal of the system (the system contains a toxic substance and you are concerned that the relief valve may leak). Application: rupture disk installed upstream of the relief valve.

2.  The system contains solids that may plug the relief valve over time. Remember, the relief valve is continuously exposed to the system. Application: rupture disk installed upstream of the relief valve.

3.  TO SAVE MONEY!  If the system is a corrosive environment, the rupture disk is specified with the more exotic and corrosion resistant material. It acts as the barrier between the corrosive system and the relief valve. Application: rupture disk installed either upstream and/or downstream of the relief valve.

Below is a capital cost comparison between combination Hastelloy C rupture disks with stainless steel relief valves and three stand-alone Hastelloy C relief valves. Again, this is based on a budget estimate in year 2001 dollars using Continental Disc Corp. rupture disks and holders and Farris Engineering relief valves.

Table 2: Cost Comparison for Rupture Disk-Relief Valve Combinations

Basis: Continental Disc	Basis: Farris Engineering
3" Ultrx Hast C Holder = $3,300	3" x 4" Hast C 26KA10-120 = $13,400
3" Ultrx Hast C Disc = $2,600 for 1st usable disk, then $870 each	3" x 4" SS 26KA10-120 = $4,300

Combination of Hastelloy C Disk and SS Relief Valve

Single Installation Total = $10,200

Total for three installations = $27,140

Three stand-alone Hastelloy C relief valves = $40,200

Summary

A stand-alone rupture disk is used when:

1. You are looking for capital and maintenance savings

2. You can afford to loose the system contents

3. The system contents are relatively benign

4. You need a pressure relief device that is fast acting

5. A relief valve is not suitable due to the nature of the system contents

A rupture disk / relief valve combination is used when:

1. You need to ensure a positive seal of the system
   
2. The system contains solids that may plug the relief valve over time
   
3. TO SAVE MONEY!  If the system is a corrosive environment, the rupture disk is specified with the more exotic and corrosion resistant material

References

1. API (www.api.org) Recommended Practice 520, "Sizing, Selection, and Installation of Pressure-Relieving Device in Refineries, Part 1-Sizing and Selection", 7^th Edition (January 2000)
2. API (www.api.org) Recommended Practice 521, "Guide for Pressure-Relieving and Depressuring Systems", 4^th Edition (March 1997)
3. ASME (www.asme.org) "Boiler and Pressure Vessel Code, Section VIII, Division 1" (1998)