151.20-5—Cargo system valving requirements.
For the purpose of adequately controlling the cargo, both under normal operating and casualty conditions, every cargo piping system shall be provided with one of the following sets of control valves and meet the requirements listed below. Cargo tanks, whether gravity or pressure vessel type, for cargoes having a saturated vapor pressure of 10 pounds per square inch gauge or less at 115 °F (105 °F if the tank is insulated) shall be provided with a valving system designated as Gravity-1. Cargo tanks, whether gravity or pressure vessel type, for cargoes which are carried below ambient temperature and whose vapor pressure is maintained at 10 pounds per square inch gauge or below shall be provided with a valving system designated as Gravity-2. Cargo tanks for cargoes which have vapor pressures above 10 p.s.i.g. at 115 °F (105 °F if tank is insulated) shall be provided with a valving system designated as Pressure-1. Cargo tanks for cargoes which have vapor pressures above 10 pounds per square inch gauge at 115 °F (105 °F if tank is insulated) and which require greater protection due to their hazardous characteristics shall be provided with a valving system designated as Pressure-2. The requirements of paragraphs (a) through (d) of this section for stop valves or excess flow valves to be fitted at tank penetrations are not applicable to nozzles at which pressure vacuum or safety relief valves are fitted.
(a) Gravity-1 (G-1).
(1)
One manually operated stop valve shall be installed on each tank filling and discharge line, located near the tank penetration.
(2)
One stop valve or blind flange shall be installed at each cargo hose connection. When a cargo hose connection is in use, it shall be provided with a stop valve; which may be part of the vessel's equipment or may be part of the shore facility and attached to the barge end of the loading hose. When a cargo hose connection is not in use, it may be secured with a blind flange.
(3)
If individual deepwell pumps are used to discharge the contents of each cargo tank, and the pumps are provided with a remote shutdown device, a stop valve at the tank is not required on the tank discharge line.
(b) Gravity-2 (G-2).
(1)
One manually operated stop valve shall be installed on each tank penetration, located as close as possible to the tank.
(2)
One remote operated, quick closing shut-off valve shall be installed at each cargo hose connection.
(c) Pressure-1 (P-1).
(1)
One manually operated stop valve and one excess flow valve shall be installed on each tank penetration, located as close as possible to the tank.
(d) Pressure-2 (P-2).
(1)
One manually operated stop valve and one excess flow valve shall be installed at each tank penetration, located as close as possible to the tank.
(2)
One remote operated quick closing shutoff valve shall be installed at each cargo hose connection when in use.
(e)
Cargo tank penetrations which are connections for gauging or measuring devices need not be equipped with excess flow or remote operated quick closing valves provided that the opening is constructed so that the outward flow of tank contents shall not exceed that passed by a No. 54 drill size (0.055-inch diameter).
(f)
The control system for any required quick closing shutoff valves shall be such that the valves may be operated from at least two remote locations on the vessel; if means of fire protection is required by Table 151.05, the control system shall also be provided with fusible elements designed to melt between 208 °F and 220 °F, which will cause the quick closing shutoff valves to close in case of fire. Quick closing shutoff valves shall be of the fail-closed (closed on loss of power) type and be capable of local manual operation. Quick closing shutoff valves shall operate from full open to full closed under all service conditions in not more than 10 seconds, without causing excessive pressure surges.
(g)
Excess flow valves, where required, shall close automatically at the rated closing flow of vapor or liquid as tested and specified by the manufacturer. The piping, including fittings, valves, and appurtenances protected by an excess flow valve, shall have a greater capacity than the rated closing flow of the excess flow valve. Excess flow valves may be designed with a bypass not to exceed 0.040-inch diameter opening to allow equalization of pressure, after an operating shutdown.
(h)
Suitable means shall be provided to relieve the pressure and remove liquid contents from cargo lines and hoses to the cargo tank or other safe location prior to effecting disconnections.