Waukesha Cherry-Burrell Products: Fittings

Polished Finish Options

Polished finish options involve three areas of discussion:

1. What Part of the Tube or Fitting is Polished?

3. Which Option is Right for Your Application?

1. What Part of the Tube or Fitting is Polished?

Tube and/or Fittings have an outside surface, O.D., and an inside surface, I.D., so there are only four possible polish/non-polish combinations. Traditionally, these are known by number:

#1	Non-polished ID	Non-polished OD
	Inside diameter has mill finish.	Outside diameter has mill finish. BPE fittings are scotch-brite finished.

#3	Polished ID	Non-polished OD
	Inside diameter is polished to a choice of: 32 Ra or 20 Ra 20 Ra +EP (electropolish) 15 Ra +EP (electropolish)	Outside diameter has mill finish. BPE fittings are scotch-brite finished.

#5	Non-polished ID	Polished OD
	Inside diameter has mill finish	Outside diameter is mechanically polished with the polish marks blended.

#7	Polished ID	Polished OD
	Inside diameter is polished to a choice of: 32 Ra or 20 Ra 20 Ra +EP (electropolish) 15 Ra +EP (electropolish)	Outside diameter is mechanically polished to a #7 with the polish marks blended.

2. Quality of the Polished Surface

Stainless steel tubing/fittings are OD polished to make the outside look shiny and bright. For the ID, polishing is more critical as this is the product-contact area. Primarily, stainless steel is polished on the inside to reduce surface roughness that micro-organisms might attach themselves to. Surface roughness is measured in units of Ra (microinches). The lower the Ra, the smoother the stainless steel. In the dairy and food industry, surface finishes have traditionally been measured by grit. Grit is the number of abrasive grains per given area. The higher the grit number, the smoother the finish.

WCB has standardized the surface finish quality referenced as Ra. Approximate grit numbers are noted below. WCB offers six internal surface finishes:

32 Ra (approx. 150 grit)

20 Ra (approx. 220-240 grit)

15 Ra (approx. 320 grit)

20 RaEP (mechanically polished to 20 Ra max, then electropolished)

15 RaEP (mechanically polished to 20 Ra max, then electropolished)

Unpolished, Mill finish

3. Which Option is Right for Your Application?

Polish Location by Code

#1

Industrial

#3

Sanitary (OD unimportant or insulated)

#5

Industrial

#7

Sanitary (OD polished for clean appearance)

Polish Finish

Mill Finish

Industrial

32 Ra

Sanitary

20 & 15 Ra

Pharmaceutical

20 RaEP

Pharmaceutical

15 RaEP

Ultra-Pure Application

Comparative Table of Surface Finish Measurements

Grit

Ra (Range)

Microinches

Micrometers

RMS

150

30-35 M.I.

32

0.81

35.5

180

20-25 MI

25

0.64

27.8

240

15-20 MI

20

0.51

13.3

320

9-11 MI

12

0.30

13.3

Microinch values expressing surface finish as a roughness average (Ra) are interchangeable with those values identified as a centerline average (CLA) or arithmetic average (AA). Measured values expressed as root mean square (RMS) will read approximately 11 percent higher than values expressed in (Ra).

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