What's The Advantage of Using Slip On Flanges

In most cases RFSO flanges are used for ease of fit up and bolt alignment although there are limitations to their use in regard to fatigue strength life, pressure, temperature, service conditions, etc..

Where the ASME B31.1 Power piping code would apply (inside a electric generating plant, or on Boiler External Piping),  Para 104.5.1, 104.5.1 Flanges — General
(A) Flanges of sizes NPS 24 and smaller, that are manufactured in accordance with ASME B16.1 and B16.5, shall be considered suitable for use at the primary service ratings (allowable pressure at service temperature) except the slip-on flanges to ASME B16.5 shall be limited in application to no higher than Class 300 primary pressure service rating. Refer to para. 127.4.4.

Para 122.1.1  (Boiler External piping)  (F) American National Standard slip-on flanges shall not exceed NPS 4. Attachment of slip-on flanges shall be by double fillet welds. The throats of the fillet welds shall not be less than 0.7 times the thickness of the part to which the flange is attached.

(H) American National Standard socket welded flanges may be used in piping or boiler nozzles provided the dimensions do not exceed NPS 3 for Class 600 and
lower and NPS 2-1?2 in Class 1500.

There are six basic flanges of which one is SO type (hub style or ring style).  It's use is determined by Code/standards, client specifications and engineering judgement. 

As far as cost I will do research but would think the SO would cost more.

Edit:  According to our purchasing agent,  slip ons are usually lower in price than weld-neck flanges.  But realize that they require two fillet welds.  I have seen their use when attaching piping to pumps and other equipment that has already been set in place so as not to put undue stress on the equipment however after tack up or welding the exterior of the SO you must disconnect and make the inside weld.

more details,email to sales2@bdpipefitting.com .visit  bepipefitting.bossgoo.com