GB/T 28778-2023 PDF English
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GB/T 28778-2023: PDF in English (GBT 28778-2023) GB/T 28778-2023
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 13.240
CCS J 16
Replacing GB/T 28778-2012
Pilot operated safety valves
(ISO 4126-4:2013, Safety devices for protection against excessive pressure - Part 4:
Pilot operated safety valves, MOD)
ISSUED ON: SEPTEMBER 07, 2023
IMPLEMENTED ON: APRIL 01, 2024
Issued by: State Administration for Market Regulation;
Standardization Administration of the People’s Republic of China.
Table of Contents
Foreword ... 3
1 Scope ... 7
2 Normative references ... 7
3 Terms and definitions ... 7
4 Symbols and units ... 9
5 Design ... 10
5.1 General requirements ... 10
5.2 Main parts and assemblies ... 11
5.3 End connection ... 12
5.4 Spring ... 13
5.5 Materials ... 13
6 Exit-factory test ... 13
6.1 Purpose... 13
6.2 General ... 13
6.3 Shell hydraulic test ... 14
6.4 Shell air pressure test ... 14
6.5 Adjustment of cold test differential pressure ... 15
6.6 Valve seat sealing test ... 15
6.7 Airtightness test ... 16
7 Type test ... 17
7.1 General ... 17
7.2 Motion performance test ... 18
7.3 Discharge test ... 20
7.4 Determination of coefficient of discharge ... 22
7.5 Rated coefficient of discharge ... 22
8 Determination of discharge capacity ... 22
9 Calculation of rated discharge capacity and calculation of flow area ... 23
10 Marking and sealing ... 23
10.1 Marking ... 23
10.2 Sealing ... 24
Bibliography ... 25
Pilot operated safety valves
1 Scope
This document specifies the design, material and structural requirements, ex-factory
tests, type tests, determination of discharge, calculation of rated displacement and flow
area, as well as marking and sealing of pilot operated safety valves.
This document applies to pilot operated safety valves with set pressure not less than 0.1
MPa and flow channel diameter not less than 4 mm.
2 Normative references
The following documents are referred to in the text in such a way that some or all of
their content constitutes requirements of this document. For dated references, only the
version corresponding to that date is applicable to this document; for undated references,
the latest version (including all amendments) is applicable to this document.
GB/T 1047, Pipework components - Definition and selection of nominal size
GB/T 12241, Safety valves - General requirements (GB/T 12241-2021, ISO 4126-
1:2013, MOD)
GB/T 36588, Safety devices for protection against excessive pressure - Common
data (GB/T 36588-2018, ISO 4126-7:2013, MOD)
3 Terms and definitions
Terms and definitions determined by GB/T 1047 and GB/T 12241, as well as the
following, are applicable to this document.
3.1
pilot operated safety valve
A safety valve that is driven or controlled by discharging medium from a pilot valve.
Note: The pilot valve controls the action of the main valve according to the pressure
of the fluid medium. When the medium pressure that keeps the main valve
closed decreases, the pilot safety valve will open. When the medium pressure
closes the pilot valve again, the pilot operated safety valve will close.
3.2
5 Design
5.1 General requirements
5.1.1 The design and structure of the pilot operated safety valve shall comply with the
requirements of GB/T 12241.
5.1.2 A guiding mechanism shall be designed to ensure the reliability of motion and
sealing.
5.1.3 The valve seat and valve body of the main valve can be integral or separate. The
separate valve seat shall be fixed on the valve body and shall not become loose when
the valve is opened or closed.
5.1.4 All external adjustment mechanisms shall be locked or sealed to prevent or
facilitate the detection of unauthorized adjustment of the pilot operated safety valve.
5.1.5 For main valves with limited opening height, an opening height limit device shall
be installed. The device shall not affect the action of the main valve. If the opening
height limit device is adjustable, the manufacturer shall fix or seal it before shipment.
The opening height of the valve shall not be less than 1 mm.
5.1.6 For pilot operated safety valves used with toxic or flammable media, the media
shall be discharged to a safe place.
5.1.7 A drain connection shall be provided at the lowest position of the main valve
where liquid may accumulate in the valve body, unless other measures are taken.
5.1.8 The design stress of pressure-bearing parts shall comply with the requirements of
relevant standards.
Note: Relevant standards include GB/T 12224, etc.
5.1.9 The materials of guide surfaces (such as guide sleeves, valve discs, valve disc
seats, and valve stems) shall be corrosion-resistant, wear-resistant, and anti-bite.
5.1.10 In the event of foreseeable damage to the connections of various components,
the discharge capacity of the pilot operated safety valve shall reach the rated discharge
capacity when the pressure of the protected equipment is less than 1.1 times the
maximum allowable pressure.
5.1.11 If the additional back pressure is higher than the pressure at the main valve inlet,
measures shall be taken to prevent the medium from flowing back.
5.1.12 In certain circumstances, an opening device shall be provided or the pilot valve
shall be applied with external pressure to verify whether important operating parts can
move freely.
Avoid using air or other gases for shell testing. However, air or other suitable gases can
be used instead of hydraulic pressure testing for shell pressure testing in the following
cases:
a) Pilot operated safety valves that are not suitable for filling with liquids in terms
of design and structure;
b) Pilot operated safety valve not allowed to be used in conditions where any trace
of water is present.
The test pressure, test duration and acceptance criteria shall be as specified in 6.3.
6.4.2 Safety requirements
Considering the dangers in the air pressure test process, preventive measures shall be
taken. Pay attention to the following factors:
a) Personnel are not allowed to approach the pilot operated safety valve during the
pressurization process;
b) During the design phase, the risk of brittle fracture under test conditions shall be
fully evaluated; when selecting the material of pilot operated safety valve for air
pressure testing, brittle fracture shall be avoided during the test;
c) When the high-pressure gas in the tank is reduced to the test pressure of the pilot
operated safety valve, it shall be noted that the temperature will drop;
d) The pilot operated safety valve undergoing air pressure test shall not be subjected
to any form of impact load;
e) Protective measures shall be taken to prevent the pressure from exceeding the
test pressure during the test.
6.5 Adjustment of cold test differential pressure
The pilot operated safety valve shall be adjusted to the cold test differential pressure or
set pressure given in the design.
If the pilot operated safety valve does not pass the shell test specified in 6.3 or 6.4, it is
not permitted to adjust the cold test differential pressure or set pressure of the pilot
operated safety valve using air or other gas as the test medium.
It is permitted to set the set pressure or cold test differential pressure by adjusting the
pilot valve alone. In this case, it shall be demonstrated that the pilot valve can enable
the main valve to reach the required set pressure at this opening sensing pressure.
6.6 Valve seat sealing test
6.6.1 General requirements
The valve seat sealing test of the pilot operated safety valve shall be carried out after
the cold test differential pressure or the set pressure has been adjusted. The test
procedure and leakage requirements shall comply with the provisions of 6.6.2 ~ 6.6.4,
or may be agreed upon by the manufacturer and the purchaser.
6.6.2 Test procedure
The sealing tests of the pilot valve and the main valve can be carried out separately.
First, close the outlet side of the pilot operated safety valve and use air (or nitrogen) as
the test medium. When the set pressure is greater than 0.35 MPa, the test pressure is
90% of the set pressure; when the set pressure is not greater than 0.35 MPa, the test
pressure is the set pressure minus 0.035 MPa. The leakage volume shall be in
accordance with the provisions of 6.6.3 and 6.6.4. When the set pressure is greater than
7.0 MPa, the leakage volume increases proportionally with the increase of pressure.
When the discharge side of the pilot valve is connected to the outlet side of the main
valve, the sealing test can measure the total leakage directly at the outlet of the main
valve. The total leakage is the sum of the leakage of the pilot valve and the main valve.
The bubble leakage measurement shall be measured using a tube with an inner diameter
of 6 mm. The outlet of the tube shall be placed in a certain volume of water, 13 mm
below the water surface and perpendicular to the water surface.
6.6.3 Maximum leakage of pilot valve
The maximum leakage of the pilot valve is:
-- Elastic sealing surface: 0 bubbles/min;
-- Metal sealing surface: 20 bubbles/min.
6.6.4 Maximum leakage of main valve
The maximum leakage of the main valve is:
-- Elastic sealing surface: 0 bubbles/min;
-- Metal sealing surface: 20 bubbles/min.
6.7 Airtightness test
The connection parts between the pilot operated safety valve and all the pressurization
and pressure relief pipes and the sensing pipes shall be subjected to air tightness test.
Use air or nitrogen as the test medium. When the set pressure is greater than 0.35 MPa,
the test pressure is 90% of the set pressure; when the set pressure is not greater than
0.35 MPa, the test pressure is the set pressure minus 0.035 MPa; the pressure holding
time is at least 1 min, and no visible leakage is allowed.
The theoretical discharge capacity is calculated in accordance with the provisions of
GB/T 36588; then, the calculated value is used together with the measured discharge
capacity at the discharge pressure to calculate the coefficient of discharge in accordance
with the provisions of GB/T 36588.
7.1.6 Design changes
When the design of the pilot operated safety valve is changed so as to affect the flow
path, opening height or performance of the pilot operated safety valve, the test shall be
re-performed in accordance with the provisions of this chapter.
7.2 Motion performance test
7.2.1 Test requirements
Pilot operated safety valves for steam media may be tested using steam, air or other
gases of known properties with a superheat of up to 10 °C. Pilot operated safety valves
for air or other gaseous media may be tested using air or other gases of known properties.
Pilot operated safety valves for liquid media shall be tested using water or other liquids
of known properties.
The deviations or limit values applicable to the motion characteristics of pilot operated
safety valves are as follows.
a) Set pressure deviation: ±3% of set pressure or ±0.01 MPa, whichever is greater.
b) If the set pressure of the pilot valve is set separately, the opening sensing pressure
of the pilot valve may be inconsistent with the set pressure of the pilot operated
safety valve. The manufacturer shall explain this and declare that the adjusted
opening sensing pressure of the pilot valve can keep the set pressure of the main
valve within the specified deviation range.
c) Opening height under excess pressure: not less than the specified value marked
by the manufacturer.
d) The opening and closing pressure difference shall not exceed the value specified
by the manufacturer, but shall be within the following specified range:
● Compressible medium: The minimum value is 2.0% of the set pressure
(not applicable to adjustable pilot operated safety valves), and the
maximum value is 15% of the set pressure or 0.03 MPa, whichever is
greater;
● Incompressible medium: The minimum value is 2.5% of the set pressure
(not applicable to adjustable pilot safety valves), and the maximum value
is 20% of the set pressure or 0.06 MPa, whichever is greater.
e) For pilot operated safety valves with limited opening height, the deviation or
limit value of the excess pressure and the opening and closing pressure
difference shall be the same as that of pilot operated safety valves of unlimited
opening height.
f) The excess pressure and opening and closing pressure difference of the adjustable
pilot operated safety valve shall be verified, and the excess pressure and opening
and closing pressure difference shall be stable at any opening height between
the minimum and maximum opening heights specified by the manufacturer.
7.2.2 Motion characteristics
The manufacturer shall specify the type of pilot valve and the motion mode of the pilot
operated safety valve.
7.2.3 Test equipment
The error of the pressure measuring instrument used in the test shall not exceed ±0.5%
of the instrument range.
7.2.4 Pilot operated safety valve for testing
The pilot operated safety valve used for testing shall be representative of the design,
pressure and specification series of the valves whose motion performance is required,
and the motion performance of the pilot operated safety valve shall be within the
inspection capability of the laboratory.
When a valve series contains no less than 7 specifications, 3 specifications of valves
shall be tested. If the series contains no more than 6 specifications, the number of
specifications to be tested can be reduced to 2.
When the specification range of a series is expanded so that the pilot operated safety
valve previously tested can no longer represent the entire specification series, the
number of specifications tested shall be increased.
The pilot valve and the main valve shall be tested together. For each specification of
the tested valve, the pilot valve shall be tested using three pressures of significant
differences. When a pilot operated safety valve of one specification needs to be tested
under 3 pressures, it can be tested using 3 springs of significant differences on 1 pilot
operated safety valve, or using 3 springs of significant differences on 3 pilot operated
safety valves of the same specification. In order to determine the satisfactory
repeatability of its performance, each test shall be carried out at least 3 times. The test
at the minimum design set pressure shall be included.
For pilot operated safety valves with only one specification but multiple set pressures,
the test shall be carried out using 4 different springs representing the operating pressure
range of the pilot operated safety valve.
For pilot operated safety valves of limited opening height, a test curve of coefficient of
discharge and valve opening height can be established.
The test shall be conducted at least 3 opening heights – the minimum opening height,
the maximum opening height and a height between the minimum and maximum
opening heights – using at least 3 test pressures at each opening height. This curve can
be used to establish the coefficient of discharge at any opening height between the
minimum and maximum opening heights.
7.3.3.4 Test pressure
Each pilot operated safety valve of each nominal size shall be tested three times; the
ratio of the absolute back pressure at the outlet to the absolute discharge pressure at the
inlet shall be less than 0.25; these tests shall be carried out under the back pressure of
discharge to the air.
For compressible media, when the ratio of the outlet absolute back pressure to the inlet
absolute discharge pressure is greater than 0.25, the discharge capacity of the main
valve is related to the ratio. The test shall be carried out between the ratio of 0.25 and
the maximum required ratio, to obtain the relationship curve between the test
displacement coefficient Kd and the ratio of the outlet absolute back pressure to the inlet
absolute discharge pressure. The curve can be extended and covered when the pressure
ratio is less than 0.25.
This curve is used to determine the coefficient of discharge at any set pressure and
excess pressure. It can also be used to obtain the coefficient of discharge under back
pressure conditions.
7.3.3.5 Allowable error of discharge test
In the discharge test method, the individual test results shall be within ± 5% of the
average value.
For the case where the test error exceeds ±5%, when the ratio of absolute back pressure
to absolute discharge pressure is greater than 0.25, this error is acceptable for the test
main valve series. The curve of coefficient of discharge generated when the ratio is
greater than 0.25 is used to describe the minimum coefficient of discharge under this
ratio.
7.3.4 Adjustment of pilot operated safety valve during test
During the test, the main valve or pilot valve shall not be adjusted. If there is any change
or deviation in the test conditions, sufficient time shall be given to allow the flow,
temperature and pressure to stabilize before reading the measured values.
7.3.5 Test records and test results
9 Calculation of rated discharge capacity and calculation of
flow area
The calculation of the rated discharge of the pilot operated safety valve and the
calculation and determination of the flow channel area shall be in accordance with the
provisions of GB/T 36588.
10 Marking and sealing
10.1 Marking
10.1.1 Markings on the main valve body
The marking can be made together with the main valve body or marked on the
nameplate, which is fixed on the main valve body. The main valve body of the pilot
operated safety valve shall at least indicate the following:
a) Nominal size DN;
b) Valve body material or code;
c) Manufacturer's name or trademark;
d) An arrow indicating the direction of medium flow;
e) Furnace batch number.
10.1.2 Markings on the pilot valve body
The valve body of the pilot valve shall at least indicate the following:
a) Serial number;
b) Valve body material code.
10.1.3 Nameplate markings
The nameplate fixed to the main valve of the pilot operated safety valve shall indicate
at least the following:
a) Manufacturing license number and marking;
b) Manufacturer name (or trademark) and product name and model;
c) Date of manufacture and product number;
...... Source: Above contents are excerpted from the PDF -- translated/reviewed by: www.chinesestandard.net / Wayne Zheng et al.
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