When selecting spring loaded safety valves, an accurate knowledge and consideration of the specific operating conditions are vital to ensure that these devices work reliably. As optimal resource use becomes an increasingly relevant criterion, plant owners focus particular attention on components that can be depended on absolutely – even at their physical limits.
Author Arne Gastberg Development Engineer, ARI-Armaturen
In the design of a safety valve, the existence of a backpressure must be considered in addition to the usual parameters for dimensioning. In this present report, the function of a safety valve is intended to be shown when a constant superimposed backpressure is present in the discharge and thus acts against the direction of opening of the safety valve.
A spring loaded safety valve is a valve which allows a defined amount of fluid to be discharged automatically – unaided by any form of energy other than the medium itself – in order to prevent a predetermined safe pressure from being exceeded. It is designed in such a way that it closes as soon as normal conditions are restored, so that no more fluid exits [2, p. 4]. As the final protective device in the technical system, it plays a special role and must be depended on to work absolutely reliably. Careful design and sizing by the systems engineer as well as expert execution of the entire installation are essential for proper functioning. As far as the safety valve itself is concerned, it is important that the spring(s) and disc (closure component) can perform their work (lift) undisturbed. If this is not the case, in other words if the required lift is restricted, impeded or blocked, delayed actuation, flutter and reduced performance could be the outcome – in short, operation outside the limits allowed by the regulations. The safety and reliability of the installation as a whole would be jeopardised. All safety valves approved under European or American regulations are subject to strict standards and component monitoring procedures. The safety valves manufactured by ARI-Armaturen are approved, for example, to DIN EN ISO 4126  and ASME VIII Div. 1 .
The balance of forces of a spring loaded safety valve can be expressed simply as
Fpressure < Fspringforce = Safety valve closed (1) or
Fpressure > Fspringforce = Safety valve open (2)
In other words, the valve opens if the pressure and hence the force underneath the disc in the pressure vessel is greater than the force which presses the disc against the seat (nozzle). A basic distinction is made here between two different types of actuation, namely sudden (rapid opening) and almost continuous (but not necessarily linear opening) [2, p. 4].
Whereas the operational behaviour of spring loaded safety valves with a built-up backpres-sure has already been the object of a detailed study in , this paper examines their operational behaviour with a preset, constant superimposed backpressure and a cold differential test pressure (CDTP) setting (Figure 1). Plant planners and owners are particularly interested in answers to the following two questions:
- How does the existence of a constant superimposed backpressure influence the design of a spring loaded safety valve?
- What effect does a constant superimposed backpressure have on the safety valve’s functionality (opening pressure difference, closing pressure difference), performance and leakproofness and to what extent does a cold differential test pressure setting influence the valve’s functional characteristic?
In the context of safety valves, we differentiate between built-up backpressure and superimposed backpressure. Amongst other things, the existence of backpressure is conditional on the use of a metal bellow. The built-up backpressure is the excess pressure in the outlet pipe of the safety valve which is built up during the blow-off phase (opening) and is therefore variable. The superimposed backpressure is the excess pressure which is already present in the outlet pipe of the safety valve before the blow-off phase begins. It can be either constant or variable (Figure 2).
If the safety valve is closed, the superimposed backpressure thus also acts on the seat against the pressure in the vessel in the closing direction:
Fsetpressure = Fspringpreload + FBPsuperimposed (3)
This additional force must be taken into account when setting the compression spring, which requires correspondingly less preload [6, p. 17]. The safety valve will otherwise be actuated at too high a pressure, namely at the sum of equation (3).
The procedure described in section background is the norm when sizing and setting American safety valves to API/ASME [6, p. 25]; it is referred to as the „CDTP setting“:
FCDTP = Fspringpreload – FBPsuperimposed (4)
According to the European regulatory framework provided by DIN EN ISO 4126 [2, p. 5], the CDTP may include a correction for backpres-sure and temperature. Extensive measurements to determine this have been carried out on an air dynamometer at ARI-Armaturen (refer to section results).
The backpressure can be compensated by installing a metal bellows (Figure 3). The optimal effect is achieved if the mean bellows diameter corresponds as closely as possible to the mean seat diameter of the safety valve. A bellows is always required if the superimposed backpressure FBPsuperimposed is variable because a variable backpressure will otherwise cause the set pressure to change constantly.
The operational behaviour must be assessed using the parameters described in the recognised standards [2, 3]. The backpressure ratio, which determines whether critical or subcritical operating conditions apply, is crucial here. Critical operation means that if the pressure behind the valve seat drops further, the mass flow no longer increases [3, p. 7]. Under subcritical flow conditions, therefore, the term must be multiplied by the correction factor Kb in order to calculate the theoretical mass flow. According to [3, p. 8 and 23], Kb can be either calculated or read off, it has a direct influence on the determination of the coefficient of discharge Kd (or Kdr) [3, p. 7ff].
The air dynamometer at ARI-Armaturen shown in figure 4 has the following specifications:
- Compressed air reservoir: 6 m³; 100 bar
- Static pressure vessel: 1.5 m³; 100 bar
- Available standard air volume: Approx. 600 Nm³
- Max. transient standard volume flow: Approx. 100,000 Nm³/h
- Backpressure vessel: 4 m³; 10.5 bar
The tested safety valve had the following characteristics:
DIN EN ISO 4126, 901 series, PN 16 DN 25/40, d0=22.5 mm, nominal lift: 5.2 mm
It was tested at a set pressure of 3 , 5 , 9 and 12 bar.
A constant pressure was then preset in the outlet pipe. This pressure was increased to the following percentages before each measurement:
Constant superimposed backpressure [%] : 0, 10, 20, 30, 50, 70
Equation (3) now yields the following total set pressures for 5 bar, for example:
Total set pressure [bar]: 5.0 , 5.5, 6.0, 6.5, 7.5 , 8.5
The safety valve has been adjusted to the stated pressure (Definition: Initial Audible). Then the constant backpressure was increased and accounted for as a percentage of each explains the measured values. To reduce the time for rebuild on an adjustment of the valve (clamping bolt) and thus the CDTP correction has been omitted. Additionally, it was kept almost nearly constant through the use of an automatic control valve in the discharge line of the backpressure to the set value. The influence of built-up backpressure was negligible. The results are accordingly plotted in section results (extract 5 bar), based on equation (3) and the set pressure should correspond to the sum of Fspringforce + FBPsuperimposed. Three measurements were performed for each setting; the mean value in each case was taken as a basis for the results shown in figure 5 to 8.
Summary of the results
The safety valve responds within the allowed limits, thus despite the presence of constant superimposed backpressure and a setting according to (3) correctly, the general function characteristic of the response is not adversely affected. Despite the constant superimposed backpressure, the valve reaches its full stroke. In the version without metal bellows, the opening pressure difference increases linearly to the backpressure ratio (Figure 6), but is within the specified range, with metal bellows it is almost constant despite increasing backpressure ratio (Figure 8). A constant backpressure and the resulting adjustment according to (4) affects the working behaviour of the safety valve in particular to the effect that the blowdown changed (it decreases linearly to the backpressure ratio (Figure 8)). This means there is a risk that the safety valve with increasing backpressure ratio or a higher opening pressure after the response no longer closes reliably and has a permanent, „creeping“ leakage. In the design and sizing of safety valves with constant backpressure of the concrete application and the operating conditions are therefore always taken into account. For this purpose, the respective manufacturer should be contacted in particular to take into account the operating limits and changing operating parameters in the selection and interpretation.
List of sources
 E. Stork; Backpressure Safety Valves, Technische Überwachung; Bd. 47 (2006), No. 7/8, July/August; Springer VDI Verlag
 DIN EN ISO 4126-1: 12.2013
 DIN EN ISO 4126-7: 12.2013
 AD 2000-A2 Specification,
Revised Version, 07.2013
 VdTÜV Component Test Sheet,
Safety Valve 663, 12.2014
 API 520 PT1 Sizing and Selection, 07.2014