Design Codes/Standards are used to design bolted flange connections. In general, these codes/standards provide guidance on design and selection of flanges, bolts, and gaskets to ensure adequate performance of the joint. Some codes/standards have implemented the concept of joint tightness. Below is a discussion of two main design codes/standards and their respective design gasket factors. Other concepts used in industry are also briefly discussed.
ASME Boiler and Pressure Vessel Code
ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 and 2 contain rules for the design and construction of unfired pressure vessels. The design of bolted flanges requires that gasket constants referred to as maintenance (m) factor and seating (y) stress be used in the calculation. The recommended values given for the gaskets listed in the code are non-mandatory. However, these constants, m and y, must be used in the code formulas unless the designer can justify the use of other values for these constants. Values for constants of specific gaskets are included in Division 1, Table 2-5.1. Additionally, gasket manufacturers publish m and y values for their own specific gasket materials and styles.
NOTE: There is currently no industry standard test to determine the m and y gasket constants, so many gasket manufacturers have developed individual test procedures based on the withdrawn ASTM F586 test method. There is no approved ASME alternative to the code that requires use of these constants.
Bolted Flange Design Methodology
This methodology is based on the Taylor-Forge approach to flange design, which did not include the concept of tightness.
A flange must be designed to create sufficient compressive load on the gasket contact area, to create an initial seal with essentially no pressure in the vessel. The gasket must conform to the flange surface and be sufficiently compressed to compensate for internal voids or spaces that could be detrimental to a seal. The gasket stress required to achieve this initial seal is considered the y constant.
The m value allows the flange designer to determine the compressive load on the gasket required to maintain tightness when the vessel is pressurized. This value is considered a multiplier or maintenance factor. This constant is intended to ensure that the flange has adequate strength and available bolt load to hold the joint together, while withstanding the effects of hydrostatic end force or internal pressure. The design intent is that the flange and bolting will hold the flanges together under pressure and exert an additional stress on the gasket of m multiplied by the internal pressure.
The designer calculates the load required to seat the gasket (Wm2) using y, and calculates load required during operation (Wm1) using m and the design internal pressure. The flange design is then based on the larger of the two calculated values.
Critical Considerations
There are some critical considerations when using the m and y in the two design equations (Wm1 and Wm2); including the fact that they:
ROTT TEST PROCEDURE
While there are no design codes or standards that are based on gasket factors derived from ROTT, ROTT is often referenced/used to describe gasket sealing performance relative to tightness class in the Americas.
Test Methodology
The ROTT test is performed on a 4-inch NPS gasket using a ROTT test rig with helium gas as the pressurized medium. The test includes two parts.
Part-A represents initial joint tightening and gasket seating. The test includes 5 main gasket stress levels in psi; 1025 (S1), 3040 (S2), 5390 (S3), 7750 (S4), and 10110 (S5) psi, respectively. At each stress level, leakage is measured at 400 psig and 800 psig.
Part-B simulates the operating conditions by performing leakage rate measurements during unload-reload cycles, which are described below
Key Outputs
There are four key outputs from the ROTT test procedure:
pmax
)
Upon initial seating, gasket tightness normally increases with increasing gasket stress. The maximum tightness parameter, Tpmax, is simply the highest level of tightness achieved during the ROTT test. Normally, the Tpmax value corresponds to the maximum gasket stress level, S5. A high Tpmax is favorable.
b
, a and Gs
)
“Gb” and “a” are obtained from the seating load sequence (Part-A) of the ROTT test. “Gb” represents the loading of the gasket at Tp =1, where Tp is the Tightness Parameter. “a” describes the rate at which the gasket develops tightness with increasing stress. For a given gasket material the “Gb” and “a” values are interdependent to determine the gasket stress. Low values of both “Gb and “a” are a favorable and they indicate that the gasket requires a lower gasket stress to achieve a given tightness level.
“Gs” is obtained from the load-unload cycles (Part-B) and is related to the operating conditions of the gaskets. “Gs” shows how sensitive the gasket is to the normal operating conditions such as vibration, shock load, pressure changes and other acts that try to reduce the gasket load and allow more leakage. A lower value of “Gs” is favorable, which indicates that the gasket is less sensitive to unloading.
CEN EN 1591
CEN has published EN1591 Flanges and Their Joint – Design Rules for Gasketed Circular Flange Connections which provides the European standard requirements for designing flanges. This document consists of four parts:
Design Methodology
The flange calculation described in CEN EN 1591 is a complex calculation that considers flange tightness. As a result, the gasket design parameters also incorporate tightness, L. Gasket manufacturers are responsible for publishing the parameters for their gaskets at the relevant test conditions.
The gasket parameters and associated test method are described in EN 13555 Flanges and Their Joints – Gasket Parameters and Test Procedures Relevant to the Design Rules for Gasketed Circular Flange Connections.
Note: the CEN EN 13555 gasket parameters only apply to Ring (IBC) gaskets, as a DN40 PN40 is the test specimen size.
QR
– creep relaxation factor, the ratio of the residual and initial surface pressures.
Definition: this factor to allows for the effect of the imposed load on the relaxation of the gasket between the completion of bolt-up and long term experience at the service temperature
min(L)
– the minimum level of surface pressure required for leakage rate class L on assembly
Definition: the minimum gasket surface pressure on assembly required at ambient temperature in order to seat the gasket into the flange facing roughness and close the internal leakage channels so that the tightness class is to the required level L for the internal test pressure
smin(L)
- the minimum level of surface pressure required for leakage rate class Lafter off-loading
Definition: the minimum gasket surface pressure required under the service pressure conditions, (i.e.) after off-loading and at the service temperature, so that the required tightness class L is maintained for the internal test pressure
smax
– the maximum surface pressure that can be safely imposed upon the gasket at the service temperature without damage
Definition: the maximum surface pressure that may be imposed on the gasket at the indicated temperature without collapse or “crush”, compressive failure, unacceptable intrusion into the bore or damage of the stressed area of the gasket such that failure was imminent
G
– unloading modulus of elasticity of the gasket
Definition: this is the additional change in thickness of the gasket or sealing element due to creep between the completion of the loading and the end of the test period