External Seals

Interna] and External Seals Mechanical seals are classified broadly as internal or external. Internal seals (Fig. 10-112) are installed with all seal components exposed to the fluid sealed. The advantages of this arrangement are (1) the ability to seal against high pressure, since the hydrostatic force is normally in the same direction as the spring force (2) protection of seal parts from external mechanical damage and (3) reduction in the shaft length required. 

External seals (Fig. 10-115) are installed with all seal components protected from the process fluid. The advantages of this arrangement are that (1) fewer critical materials of construction are required, (2) installation and setting are somewhat simpler because of the exposed position of the parts, and (3) stuffing-box size is not a hmiting factor. Hydraulic balancing is accomphshed by proper proportioning of the seal face and secondary seal diameters. 

Throttle bushings (Fig. 10-117) are commonly used with single internal or external seals when sohds are present in the fluid and the inflow of a flushing fluid is not objectionable. These close-clearance bushings are intended to serve as flow restric tions through which the maintenance of a small inward flow of flushing fluid prevents the entrance of a process fluid into the stuffing box. 

Circulate vessel contents via external, seal-less pump... 

Another method is to install an external labyrinth with purge provisions on the bearing housings of a machine that has the necessary space to accommodate the external seal. 

Removable-Bundle, Externally Sealed, Floating-Head Exchanger... 

Certain devices may be obtained which are factory-sealed —that is, interconnecting wiring is sealed by the manufacturer where it enters/exits enclosures. These devices do not require an additional (external) seal, and often can be utilized to advantage in lessening installation time and reducing space requirements (for an external seal). 

The average unbalanced external seal is good for pressures of about 30 psig, while the balanced design will handle 150 psig. Special designs will handle much higher... 

When a mechanical seal is specified, it shall be located at the cover plate to seal the vapor in the supply tank or vessel. Mechanical seals normally seal vapor however, they shall be designed to operate in liquid in the event of tank or vessel overfilling. With purchaser approval, the vendor may supply a seal pressurized by flush from the pump discharge or from an external source. External seal flush must be compatible with the liquid being pumped. The seal chamber shall have provisions for a high point vent. 

This e.xception in the Code offers potential savings by the elimination oi costly external seal fittings, and the... 

MOUNTING THREAD COLUMN MICA INSULATOR EXTERNAL SEAL ... 

A tube ky Fig. 708) is inserted in the hole and externally sealed around the lower end with a com-... 

Fluorinated hydrocarbons of fatty consistency can be used for lubricating metal surfaces and taps, with an external seal of piceine. Packing rings of lead, soft iron, or copper are used when connecting metal apparatus. Litharge in glycerol is a suitable cement. 

The fluorspar tube is connected to the quartz sections of the apparatus with ground joints, which are externally sealed with an asbestos-waterglass mixture. 

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