(4) Design guide

(a) Selection method
• Single seals and double seals
The sealing face (sliding surface) of a mechanical seal, except one incorporating special consideration, is generally arranged to perform lubrication and sealing by means of the fluid to be sealed. It is thus required that the region all the way from the lubricating and sealing arrangements through to the sealing face be covered with a liquid film at all times.
For this reason, double seats (i.e. two pair of single seals) should be adopted and a sealing liquid at 0.04 to 0.19 MPa {0.5 to 2 kgf/㎠G} should be injected into (seal) the space between the two pairs of seals from a different source, if any of the following events is applicable to the fluid to be sealed:
(i) where gas is sealed as in a blower or as is the case with an upper seal for a vertical type agitator;
(ii) where, in case of a pump, the liquid film on the sealing face boils into dry contact due to the influences of high temperature, the sliding heat of the sealing face, the agitating heat of the mechanical seal, and the like; or
(iii) where greater safety is required for an explosive or toxic fluid.
For double seals, this arrangement to inject the sealing liquid (for sealing) should serve to perform cooling and lubrication.
(b) Materials for unbalanced/balanced type & sealing face
The materials involved are classified according to:
(i) PV factor (the product derived from the pressure, “P,” acting on the sealing face and the sliding speed, “V”)
(ii) Characteristics of the fluid to be sealed
(iii) Combined materials of the sealing face
(iv) Economic efficiency
(c) Calculation method for PV factors
　PV＝（K・Pi＋Fs）×π・D・N／60×1000
P: Tightening stress (kgf/㎠) V: Sliding speed (m/sec)
K: Balance ratio (unbalanced type: 1.2, balanced type: 0.65〜0.85)
Pi: Fluid pressure (kgf/㎠) Fs: Spring force (kgf)
D: Sealing face outer diameter (mm) N: Revolutions (RPM)
(d) Stuffing box design
• JIS B 2405–1993 (Mechanical seals – General requirements)
• SMA 131 (Standard for ship-use mechanical seals)
The dimensions of our mechanical seals are designed in accordance with the above standards.