2 Questions
1) Can NPS 12 class 150 BW swing check or gate valve be welded directly to a BW 90 deg. LR elbow?
2) Can NPS 12 class 150 BW swing check or gate valve be welded to each other?
 

Yes to both.  Fitting to fitting assemblies are okay.

Unless I had no room, I woulnd't weld the two valves together, just for the fact that if you had to cut one out and replace it, you would have to replace both.

Also, regarding the two valves, consider the potential to trap pressurized gas/liquid, and ensure that you can positively vent that cavity.

Thanks.  Good points.

It wouldn't be considered "good practice".
No room for bad weld repair and vorticies off the elbow may cause excessive movements of the swinging plate.  Check would normally go downstream of the blocking valve.

What would you be doing, if you knew that you could not fail?

This is getting off the original questions.
But "BigInch's answer "Check would normally go downstream of the blocking valve" needs some clarification.

Normally on a Pump discharge line (in a refinery, chemical plant,etc) that has both a Check valve and a Block Valve the Check Valve is upstream of the Block Valve.

BigInch may be thinking of another application or he misspoke.

Yes, I agree w/ BigInch on the drawback of placing the check valve next to the elbow.  If space was tight, what would be a min. spacing from elbow...eg. 1D, 2D, 3D?  I guess the right answer is the max. the space allows. Some instr. require 5D straight length.
Offtopic, pennpiper is correct...on pump disch. check is upstream of isolation valve.
Thanks.

If you weld it - don't you enhance the chance of distortion and valve not seating correctly??  Just a dumb structural who has run into this problem before....

Closing a block downstream of a check valve traps pressure between the check and block, so we wouldn't use that configuration without installing a 1" vent between them as it is not advisable to dismantle for maintenance what would be the pump unit upstream of that assembly as you're looking into the rear end of a live pressure containing check valve.  If the check were downstream of the block, you could dismantle the upstream pump unit for maintenance with the confidence of positive isolation that a block valve can provide.  

Miketheengineer is also correct.  I did not mention the increased possibility of creating heat damage to the valve.  We ask for WE valves to be supplied with pups installed on both ends at the valve mfgr's shop to eliminate any possibilities of damaging the valves during field weld-ups.

What would you be doing, if you knew that you could not fail?

So I have to ask ...IF the valve was manufactured to be welded in, wouldn't the valve be design to deal with the welding and NOT distort?
 Also I agree with adding venting between the block valve and check, but I've seen where a 1/4" Dia hole was drilled thru the clapper in the check to eliminate the issue of "trapped pressure". FYI - I always wondered wheather seat on a check valve would hold tight enough to cause a problem ...since it wasn't designed for bubble tight shut-off!??

"Closing a block downstream of a check valve traps pressure between the check and block, so we wouldn't use that configuration without installing a 1" vent between them as it is not advisable to dismantle for maintenance what would be the pump unit upstream of that assembly as you're looking into the rear end of a live pressure containing check valve."

Thank you!!

I don't know how many times in a P&ID review I have had to argue this point with a client.  "But it's an extra valve" or "Can't we just bolt them together?" or "won't the check valve leak enough?".  

My response is "do you want to have to explain to the person that gets sprayed with (fill in the blank) that you saved an extra bleed valve this way?"

Or blasts the workplace with colorless, oderless gas Z.

I say, "Oh!  Are you sure?  That's the kind of answer I'd expect from a contractor, or anybody else that's not going to be concerned with operating and maintaining this thing for the next 50 years.  Let's call someone from your O&M department to come in here and see what they think of your idea".

Don't clients love to be "penny wise and pound foolish".   

What would you be doing, if you knew that you could not fail?