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API REPORT 5 Document Information:
Title
Study of Pressure Drop and Closure Forces in Velocity-Type Subsurface Safety Valves
American Petroleum Institute
Publication Date:
Jul 1, 1977
Scope:
1. INTRODUCTION
Subsurface safety valves (SSSVs) are required by law in most offshore
producing wells. The purpose of the valves is to shut off well flow in
the production tubing below the mudline in the event disasters, such
as explosions or fires, disable surface shutdown devices. Several
types of SSSVs are used, including those which are controlled from the
surface by hydraulic fluids, pressure sensing valves, and differential
pressure or fluid velocity actuated valves. This report deals only
with velocity actuated SSSVs.
Actuation of the velocity-type SSSV is based on a simple force balance
principle. Loss of pressure above a valve increases the flow rate
through the valve and also the pressure loss across the valve. For
subcritical flow, the pressure loss across a restriction, such as the
bean (or choke) used in a safety valve, is proportional to the flow
rate of fluids. The safety valve is held open by spring and seal
gripping forces which together are greater than the opposing resultant
well fluid forces generated by normal production rates. However, for
higher than normal production rates corresponding to loss of
tubinghead back-pressure, the net well fluid forces become great
enough to overcome the spring and seal gripping forces and to actuate
valve closure. The consequences of incorrect valve sizing are either
premature closures, which result in lost production and operator
expense, or loss of protection from using a valve which cannot be
closed by well flow rates corresponding to disaster conditions.
Before recent API safety valve standards were written, functional
testing procedures and selection of manufacturing tolerances on
critical valve components were left to the discretion of valve
manufacturers. New API standards1 and recommended practices2 have been
written by the API Committee on Standardization of Offshore Safety and
Anti-Pollution Equipment (OSAPE). These documents Provide
manufacturing tolerances and a formal procedure for functional and
performance testing.
Current recommendations for valve type and spring and choke size for
each well condition are made using technology based on single-phase
flow theory. Since most valves operate under gas-liquid flow
conditions, the development of improved multiphase flow predictions
was recognized as a high potential area for safety valve improvement.
As a result, the API Offshore Safety and Anti-Pollution Research
Committee (OSAPR) awarded a research grant to The University of Tulsa
to study multiphase flow through safety valves and chokes. The purpose
of this research was to develop correlations for predicting pressure
drop across a SSSV occurring during multiphase flow as a function of
variables such as gas and liquid flow rates, bean or choke size,
gas-liquid ratio and average pressure. The study was performed
specifically for 2-3/8 in. nominal Otis J and Camco A-3 valves. The
results of this study, which was OSAPR Project No. 1, were reported in
September, 1976.3
The design procedure to be followed when selecting a velocity-type
SSSV for a particular well is illustrated in Fig. 1.1, which is a
logic diagram of the computer program described in API RP-14B.2 Steps
1-3 are based on measured well flow data and are required to predict
the productivity or inflow performance of the well. Steps 4-6 are used
to determine the bean size required to produce the desired pressure
drop across the valve for the selected closure flow rate. Once the
bean size and pressure drop are determined, it is then necessary to
select the valve components which will allow the SSSV to close at the
pressure drop calculated in Step 6. This is illustrated in Step 7 and
involves selecting the spring force which balances the pressure force
tending to close the SSSV at the selected flow rate. The results of
Project No. 1 can be used to perform the calculations highlighted in
Steps 1 and 6.
A very important step in the total design procedure, Step 7, was not
considered in Project No. 1. This involves predicting the pressure
drop across the valve at which closure will occur when the valve is
equipped with a particular bean and spring combination. Valve closure
occurs when the fluid forces are sufficient to overcome spring forces
and seal gripping forces or friction. Spring forces can be estimated
from a knowledge of the spring constant and as a function of the
number of spacers used to impart an initial compression in the spring.
Prior to this study very little was known about the nature of the seal
gripping and other frictional forces.
Previous closure test data were obtained using single-phase liquid or
gas as the flowing fluid. Consequently, in order to improve this step
in the design procedure a proposal was submitted by The University of
Tulsa to the API OSAPR committee in November, 1975. This proposal was
funded in February, 1976 and the existing experimental facility was
used to obtain multiphase flow valve closure data to solve the
problem. The SSSVs selected for testing were the nominal 2-3/8 in.
Otis J. and Camco A-3 valves. The valves used were furnished by the
manufacturers. Several combinations of bean size and spring force were
utilized for each valve. Since the actual valves were used, the
pressure drop occurring across the valve and its locking mandrel was
measured and used to develop the design equations.
As a consequence of the procedure used to conduct a closure test, many
additional multiphase flowing pressure drop data points were obtained
for each bean size for both valves. These data points were combined
with those obtained in Project No. 1 to develop improved pressure drop
correlations. Since these data were obtained using actual valves, it
is felt that the improved pressure drop correlations combined with the
closure equations developed in this study represent a significant
improvement in the overall SSSV design procedure.
The purpose of this report is to present find results of the OSAPR
Project No. 5 research project. Future sections will describe the
experimental equipment and testing procedure (Section II), the
experimental data obtained (Section III), the development and
evaluation of the closure correlation (Section IV), the revised
pressure drop correlations (Section V), and the conclusions and
recommendations (Section VI). Example design problems to illustrate
the application of the results are presented as an appendix.
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