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A somewhat recent development is the use of magnetic force to obtain a face-loading action. Magnetic seals have provided reliable service under a variety of fluids and severe operating conditions. Some of the design advan-tages claimed are that magnetic seals are compact and lighter， provide aneven distribution of sealingforce， and are easy to assemble. Figure 13-28 shows a simple magnetic seal.
Shaft sealing elements can be split up into two groups. The first may becalled pusher-type seals and includes theO-ring，.-ring，U-cup， and wedgeconfigurations. The second group are Bellow-type seals， which differ from the pusher-type seals in that they form a static seal between themselves and the shaft. Figure 13-29 shows some typical pusher-type seals.
A typical mechanical contact shaft seal has two major elements， as seen in Figure 13-30. These are the oil-to-pressure-gas seal and the oil-to-unconta-minated-seal-oil-drain seal or breakdown bushing. This type of seal will normally have buffering via a single ported labyrinth located inboard ofthe seal and a positive shutdowndevice， which will attempt to maintain gas

Figure 13-2.. Simple magnetic-type seal.

pressure in the casing when the compressor is at rest and seal oil is not beingapplied. For shutdown， the carbon ring is kept tightly sandwiched between the rotating seal ring and stationary sleeve with gas pressure to prevent gas from leaking out when no oil pressure is available.
In operation， seal oil pressure is held at a differential of 35.50 psid (2.4.3.5 bar) over the process gas pressure which the seal is sealing against. This high-pressure oil can be seen entering in the top in Figure 13-30 and completely fills the seal cavity. Some of the oil (a relatively small percentage， ranging from 2 to 8 gpd per seal depending on machine size) is forced acrossthe carbon ring seal faces， which are sandwiched between the rotating seal ring (rotating at shaft velocity) and the stationary sleeve (nonrotating andforced against the carbon ring by a series of peripheral springs). Therefore， the actual rotative speed of the carbon ring can be anywhere between zero rpm and full rotational speed. Oil crossing these seal faces contacts theprocess gas andis，thus， ""contaminated oil.''

The majority of the oil flows out of the uncontaminated seal oil drain after taking a pressure drop from design seal oil pressure to atmospheric pressure across the breakdown bushing. An orifice is placed in parallel with the break-down bushing to meter the proper amount of oil flow for cooling. The contaminated oil leaves through the drain to a degasifier for purification.
The bearing oil drain can be either combined with the uncontaminated sealoil drain or kept separate; however， a separate system will increase bearing span and lower critical speeds.
.echanical Seal Selection and Application
The following is a list of factors that have proven to be helpful in seal system design and selection:
1. Product

2. Seal environment

3. Seal arrangement

4. Equipment

5. Secondary packing

6. Seal-face combinations

7. Seal gland plate

8. Main seal body

Product
The physical and chemical properties of the liquid being sealed will placeconstraints upon the type of seal arrangement， the materials of construction， and the seal design that can be used.
Pressure. The relative pressures of the material to be sealed affect the decision of whether to use a balanced or unbalanced seal design. Pressure also affects the choice of face material because of the seal-face loading.
If the servicehappens to be below atmospheric pressure， then special considerations are required to seal the material effectively. Most unbalanced seal designs are applicable up to 100 psig (7 Bar) stuffing-box pressure. Atmore than 100 psig (7Bar)， balanced seals should be used.
Seal manufacturers base their seal-face combination designs onP主 rat-ings. These are the multiple of the face load (P) and the sliding velocity (主)of the faces. The maximumP主rating for an unbalanced seal is about200，000and about2，250，000 for a balanced seal.
　
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