Electrical Area Classification in Coal-fired Power Plants

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Hom e (http ://w w w .p ow e r m ag.com )/ Coal (http ://w w w .p ow e r m ag.com /cate gor y/coal /)/El e c t r i c a l A r e a Cl a s s i f i c a t i o n i n Co a l -Fi r e d Po w e r Pl a n t s

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El e c t r i c a l A r e a Cl a s s i f i c a t i o n i n Co a l -Fi r e d Po w e r Pl a n t s10/07/2013 | Ram K.Saini,P.E.

El e c t r i c p o w e r p r o d u c t i o n f r o m c o a l i s o n a s t e e p r i s e i n m a jo r d e v e l o p i n g c o u n t r i e s ,i n c l u d i n g Ch i n a ,In d i a ,In d o n e s i a ,So u t h A f r i c a ,a n d Vi e t n a m ,a l b e i t d e c l i n i n g i n d e v e l o p e d c o u n t r i e s s u c h a s t h e Un i t e d S t a t e s .S h o r t f a l l s i n c o a l p r o d u c t i o n h a v e b e e n r e p o r t e d i n s o m e o f t h e s e c o u n t r i e s ,b u t t h e s e i s s u e s a r e b e i n ga d d r e s s e d b y i n c r e a s i n g c o a l p r o d u c t i o n ,a s w e l l a s b y u s e o f i m p o r t e d c o a l f r o m o t h e r c o u n t r i e s .Th e m a jo rc o n c e r n s o f Pu l v e r i ze d Co a l (PC)o r Ci r c u l a t i n g Fl u i d i ze d Be d (CFB)c o m b u s t i o n t y p e c o a l -f i r e d p o w e r p l a n t sa r e f i r e s a n d e xp l o s i o n s i n h a za r d o u s a r e a s a n d g l o b a l w a r m i n g a n d o t h e r e n v i r o n m e n t a l i s s u e s .Fi r e s a n de xp l o s i o n s h a v e c a u s e d a s i g n i f i c a n t n u m b e r o f d e a t h s a n d i n ju r i e s t o p o w e r p l a n t s t a f f .Be s i d e s o t h e ri g n i t a b l e m a t e r i a l s u s e d i n c o a l -f i r e d p o w e r p l a n t s ,c o a l d u s t h a s b e e n i d e n t i f i e d a s t h e m a jo r s o u r c e o f t h e s ef i r e s .Fo r c o a l t o r e m a i n a s a v i a b l e f u e l i n p o w e r p r o d u c t i o n i n t h e p o w e r g e n e r a t i o n i n d u s t r y ,p r o t e c t i v em e a s u r e s a r e r e qu i r e d d u r i n g e n g i n e e r i n g ,d e s i g n ,c o n s t r u c t i o n a n d o p e r a t i o n o f t h e c o a l -f i r e d p o w e rp l a n t s ,p a r t i c u l a r l y i n e l e c t r i c a l a r e a s t h a t a r e o f t e n a s o u r c e o f i g n i t i o n .

The Fir e Tr iangl e

Page 1 of 23Electrical Area Classification in Coal-Fired Power Plants

2016-01-26http://www.powermag.com/electrical-area-classification-in-coal-fired-power-plants/?prin...

Th e f i r e t r i a n g l e (Fi g u r e 1 )i s a w e l l -kn o w n t o o l t h a t i l l u s t r a t e s t h e t h r e e c o n d i t i o n s t h a t m u s t b e p r e s e n t f o ra f i r e o r e xp l o s i o n t o o c c u r a t a p a r t i c u l a r l o c a t i o n :Fl a m m a b l e o r c o m b u s t i b l e m a t e r i a l m u s t b e p r e s e n t ;t h em a t e r i a l m u s t b e m i xe d w i t h a i r i n t h e p r o p o r t i o n s a n d c o n c e n t r a t i o n t o f o r m a c o m b u s t i b l e m i xt u r e ;a n d t h ei g n i t i o n s o u r c e m u s t s u p p l y e n o u g h e n e r g y t o i n i t i a t e c o m b u s t i o n .A s p a r k o r f l a m e i s n o t n e c e s s a r y ,a st e m p e r a t u r e a l o n e c a n s u p p l y t h e e n e r g y t o c a u s e i g n i t i o n o f t h e m i xt u r e .

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T h e Fir e T r ian gl e .Oxygen,fuel,and sufficentheatm ustbe presentforignition.Source:W ikim edia

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Th e e n e r g i e s r e qu i r e d t o i g n i t e v a r i o u s g r o u p s o f c o m b u s t i b l e s u b s t a n c e s h a v e b e e n p r o v e n b ye xp e r i m e n t a t i o n .Th e c o n c e p t s i n c l u d e d i n t h e f i r e t r i a n g l e h a v e b e e n s p e c i f i e d i n c o d e s a n d s t a n d a r d si s s u e d b y v a r i o u s w o r l d o r g a n i za t i o n s p r o v i d i n g g u i d a n c e i n t h e d e s i g n o f e l e c t r i c a l s y s t e m s ,s e l e c t i o n o fe qu i p m e n t ,a n d c o n s t r u c t i o n a n d o p e r a t i o n o f p o w e r p l a n t f a c i l i t i e s .

Hazar d ous Cl assifie d Locations

Th e N a t i o n a l El e c t r i c a l Co d e (N EC)i n Un i t e d S t a t e s a n d In t e r n a t i o n a l El e c t r o t e c h n i c a l Co m m i s s i o n (IEC)i no t h e r c o u n t r i e s d e f i n e h a za r d o u s a r e a l o c a t i o n s a s t h o s e a r e a s ,w h e r e f i r e o r e xp l o s i o n h a za r d s m a y e xi s td u e t o p r e s e n c e o f c o m b u s t i b l e d u s t o r i g n i t a b l e f i b e r s ,f l a m m a b l e g a s e s o r v a p o r s ,a n d f l a m m a b l e l i qu i d s .El e c t r i c a l e qu i p m e n t c a n b e c o m e a s o u r c e o f i g n i t i o n i n t h e s e v o l a t i l e a r e a s .



H a za r d o u s a r e a s a r e c l a s s i f i e d b y t y p e s ,d i v i s i o n o r zo n e ,a n d n a t u r e /g r o u p .Th e r e a r e t h r e e t y p e s o fh a za r d o u s c o n d i t i o n s ,Cl a s s I Ga s a n d Va p o r ,Cl a s s II Du s t ,a n d Cl a s s III Fi b e r s a n d Fl y i n g s .H a za r d o u sa r e a l o c a t i o n s a r e c a t e g o r i ze d b y t w o m e t h o d s p e r N EC:

Cl a s s I,II,a n d III,a n d Di v i s i o n 1 a n d 2 M e t h o d :A r t i c l e 500 Cl a s s 1 ,Zo n e 0,1 a n d 2 M e t h o d :A r t i c l e 505 a n d IEC/EN 6007 9-1 0.

Th e c l a s s a n d d i v i s i o n m e t h o d h a s t r a d i t i o n a l l y b e e n u s e d i n t h e Un i t e d S t a t e s .Th e c l a s s a n d zo n e m e t h o d h a s b e e n u s e d i n o t h e r c o u n t r i e s i n a c c o r d a n c e w i t h IEC s t a n d a r d s .

Th e r e a r e t w o t y p e s o f d i v i s i o n s u n d e r Cl a s s Ic o n d i t i o n s :

W h e r e i g n i t a b l e c o n c e n t r a t i o n s o f f l a m m a b l e g a s e s ,v a p o r s o r l i qu i d s c a n e xi s t a l l o r f o rs o m e t i m e u n d e r n o r m a l o p e r a t i n g c o n d i t i o n s

W h e r e i g n i t a b l e c o n c e n t r a t i o n s o f f l a m m a b l e g a s e s ,v a p o r s o r l i qu i d s a r e n o t l i ke l y t o e xi s tu n d e r n o r m a l o p e r a t i n g c o n d i t i o n s

S i m i l a r l y ,t h e r e a r e t w o t y p e s o f d i v i s i o n s u n d e r Cl a s s IIc o n d i t i o n s :

W h e r e i g n i t a b l e c o n c e n t r a t i o n s o f c o m b u s t i b l e d u s t c a n e xi s t a l l o r f o r s o m e t i m e u n d e rn o r m a l o p e r a t i n g c o n d i t i o n s

W h e r e i g n i t a b l e c o n c e n t r a t i o n s o f c o m b u s t i b l e d u s t a r e n o t l i ke l y t o e xi s t u n d e r n o r m a lo p e r a t i n g c o n d i t i o n s

Zo n e s d e f i n e d p e r IEC s t a n d a r d s a r e a s f o l l o w s :

Zo n e 0.W h e r e i g n i t a b l e c o n c e n t r a t i o n s o f f l a m m a b l e g a s e s ,v a p o r s o r l i qu i d s c a n e xi s t a l l o f t h e t i m e o rf o r l o n g p e r i o d s a t a t i m e u n d e r n o r m a l o p e r a t i n g c o n d i t i o n s .

Zo n e 1 .W h e r e i g n i t a b l e c o n c e n t r a t i o n s o f f l a m m a b l e g a s e s ,v a p o r s o r l i qu i d s c a n e xi s t s o m e o f t h et i m e u n d e r n o r m a l o p e r a t i n g c o n d i t i o n s .

Zo n e 2.W h e r e i g n i t a b l e c o n c e n t r a t i o n s o f f l a m m a b l e g a s e s ,v a p o r s o r l i qu i d s a r e n o t l i ke l y t o e xi s tu n d e r n o r m a l o p e r a t i n g c o n d i t i o n s .

N a t u r e /Gr o u p s o f H a za r d o u s S u b s t a n c e s a r e :

Gr o u p A ,B,C a n d D i n Cl a s s Il o c a t i o n s Gr o u p E,F a n d G i n Cl a s s IIl o c a t i o n s

Com bustibl e and Fl am m abl e M ate r ial s in a Coal -Fir e d Pow e r Pl ant

M a n y t y p e s o f c o m b u s t i b l e a n d f l a m m a b l e m a t e r i a l s c a n b e p r e s e n t i n c o a l -f i r e d p l a n t :



Coal .Th e m a i n c o m b u s t i b l e f u e l .In a d d i t i o n ,c o a l d u s t i s a m a jo r s o u r c e o f d u s t e xp l o s i o n s i n a c o a l -f i r e dp o w e r p l a n t .It c a n c a u s e p r i m a r y e xp l o s i o n w h e n t h e r i g h t c o n c e n t r a t i o n o f f i n e l y d i v i d e d d u s t ,s u s p e n d e di n a i r ,i s e xp o s e d t o a s u f f i c i e n t s o u r c e o f i g n i t i o n c a u s i n g c o m b u s t i o n .If t h i s p r i m a r y e xp l o s i o n o c c u r s ,a d d i t i o n a l a v a i l a b l e d u s t c a n d i s p e r s e a n d s e c o n d a r y e xp l o s i o n s c a n s p r e a d t h r o u g h o u t t h e f a c i l i t y .

Pr op ane.Us e d a s s t a r t -u p f u e l .It c a n b e p r e s e n t i n g a s c y l i n d e r s ,p i p i n g t o i g n i t i o n b u r n e r a n d t oc o m b u s t o r ,g a s s h u t -o f f v a l v e ,g a s c o n t r o l v a l v e b l o c k,g a s -r e l i e f v a l v e .

Natur al gas.A s s t a r t -u p o r a l t e r n a t e f u e l .Ga s c a n b e p r e s e n t i n g a s c o m p r e s s o r s t a t i o n ,f i l t e r a n d s c r u b b e rs t a t i o n ,g a s p r e -h e a t e r a r e a s ,g a s s h u t -o f f v a l v e a n d f i l t e r ,g a s c o n t r o l v a l v e b l o c k,g a s r e l i e f v a l v e ,a n d g a sp i p i n g .

Fue l oil .A s s t a r t -u p o r a l t e r n a t e f u e l .Ca n b e p r e s e n t i n o i l t a n ks ,f u e l u n l o a d i n g a n d f o r w a r d i n g p u m ps t a t i o n ,f u e l o i l b o o s t e r p u m p a n d l e a ka g e t a n k,f u e l o i l c o n t r o l v a l v e b l o c k a n d f u e l o i l p i p i n g t o b u r n e r s .Th ef u e l o i l b e c o m e s f l a m m a b l e w h e n h e a t e d a b o v e i t s f l a s h p o i n t .

Hyd r oge n.A s c o o l i n g m e d i u m f o r g e n e r a t o r c o o l i n g a n d r e l e a s e f r o m DC s y s t e m b a t t e r i e s .A l s o p r e s e n t i nh y d r o g e n g a s u n i t ,s e a l o i l u n i t ,c o n t r o l c u b i c l e a n d p i p i n g .

Am m onia.A qu e o u s a m m o n i a i s u s e d f o r n i t r o g e n o xi d e e m i s s i o n r e d u c t i o n b y s e l e c t i v e c a t a l y t i c r e d u c t i o n(SCR)o r s e l e c t i v e n o n -c a t a l y t i c r e d u c t i o n (S N CR)m e t h o d .In c l u d e s a m m o n i a s t o r a g e a n d p i p i n g .

Sour ce s ofIgnition:El e ctr ical Equip m e nt

Th e r e a r e 3 w a y s i n w h i c h e l e c t r i c a l e qu i p m e n t c a n b e c o m e a s o u r c e o f i g n i t i o n :

Ar cs and sp ar ks.Pr o d u c e d b y t h e n o r m a l o p e r a t i o n o f e qu i p m e n t ,e .g .,m o t o r s t a r t e r s ,c o n t a c t o r s ,a n ds w i t c h e s c a n i g n i t e a c o m b u s t i b l e o r a f l a m m a b l e m a t e r i a l .

High te m p e r atur e.S o m e h e a t p r o d u c i n g e qu i p m e n t ,s u c h a s l i g h t l a m p s a n d l i g h t i n g f i xt u r e s ,e l e c t r i cm o t o r s a n d h e a t e r s c a n i g n i t e f l a m m a b l e a t m o s p h e r e s i f t h e y e xc e e d t h e i g n i t i o n t e m p e r a t u r e o f t h eh a za r d o u s m a t e r i a l .

El e ctr ical e quip m e ntfail ur e .S h o r t i n g o f a t e r m i n a l c o u l d s p a r k i g n i t i o n .

El e c t r i c a l e qu i p m e n t c a n c a u s e e xp l o s i o n s i n c o a l d u s t a t m o s p h e r e s w h e n l o c a t e d i n c o a l c o n v e y o r g a l l e r i e s ,t r a n s f e r t o w e r s ,h e a d c h u t e ,f l o p g a t e b u n ke r s ,c o a l p r e p a r a t i o n s y s t e m /c r u s h e r h o u s e ,c o a l s i l o s ,a n d f e e ds y s t e m ,t o t a l l y e n c l o s e d p o r t i o n s o f c o a l h a n d l i n g c o n v e y o r s ,c o a l h a n d l i n g b e l o w g r a d e c o n v e y o r s y s t e m s ,c o a l c h u t e s ,d u s t c o l l e c t o r s ,a n d o t h e r l o c a t i o n s w h e r e d u s t c a n s e t t l e ,h i d d e n c o n c e a l e d a r e a s ,a n d w h e r eo t h e r c o m b u s t i b l e a n d f l a m m a b l e m a t e r i a l s e xi s t .

S i m i l a r l y ,e l e c t r i c a l e qu i p m e n t c a n c a u s e e xp l o s i o n s i n p r o p a n e g a s ,n a t u r a l g a s ,f u e l o i l ,h y d r o g e n g a s a n da m m o n i a g a s a r e a s ,w h e r e f l a m m a b l e c o n d i t i o n s e xi s t .



Se l e ction and Instal l ation ofEl e ctr ical Equip m e ntfor Hazar d ous Locations

Ig n i t i o n s ,c o m b u s t i o n a n d e xp l o s i o n w i t h i n e l e c t r i c e qu i p m e n t a n d c i r c u i t w i r i n g e n c l o s u r e s i n h a za r d o u sa r e a s a r e d u e t o h o t s u r f a c e s a n d a r c s /s p a r ks ,o r c o m b u s t i b l e m a t e r i a l e n t e r i n g t h e e n c l o s u r e s .Th e i d e a le l e c t r i c a l e n c l o s u r e s h o u l d b e i n s t a l l e d a n d r e m o v e d e a s i l y ,p r o t e c t t h e e qu i p m e n t ,a l l o w t h e c o m p o n e n t sh o u s e d i n s i d e i t t o b e a c c e s s e d e a s i l y ,a n d r e s i s t a n d p r e v e n t f i r e a n d e xp l o s i o n h a za r d s .N FPA 7 0,N FPA 7 9,N EM A 250,UL 50,UL 508A ;Ca n a d i a n S t a n d a r d A s s o c i a t i o n CS A 22.2,a n d IEC 60529 p r o v i d e s g u i d a n c e i n t h es e l e c t i o n o f s p e c i f i c t y p e o f e n c l o s u r e t o m a t c h t h e e n v i r o n m e n t a l p r o t e c t i o n .

Equ i p m e n t p r o t e c t i o n m e t h o d s i n c l u d e f l a m e p r o o f ,e n c a p s u l a t i o n ,o i l e m e r s i o n ,d u s t i g n i t i o n p r o o f ,d u s tt i g h t ,p o w d e r f i l l e d ,n o n -s p a r ki n g ,n o n -i n c e n d i a r y a n d h e r m e t i c a l l y s e a l e d c o n s t r u c t i o n .M a n y o f t h e s em e t h o d s a p p l y t o A C-p o w e r e d c i r c u i t s ,b u t f e w a r e u s e d f o r i n s t r u m e n t w i r i n g c i r c u i t w i r i n g .

W h e n i n s t a l l a t i o n s a r e n o t e xp l o s i o n p r o o f o r i n t r i n s i c a l l y s a f e ,p r e s s u r i za t i o n i s o f t e n u s e d t o m a i n t a i n t h ec l a s s i f i e d a r e a s a f e t y .W i r i n g a n d e n c l o s u r e s a r e p r o t e c t e d u s i n g a p o s i t i v e p r e s s u r e m a i n t a i n e d w i t h i n t h ee n c l o s u r e ,ju n c t i o n b o xe s a n d c o n d u i t a s p e r A N S I/N FPA 496.

In t r i n s i c a l l y s a f e c i r c u i t s i n w h i c h a n y s p a r k o r t h e r m a l e f f e c t i s i n c a p a b l e o f c a u s i n g i g n i t i o n o f a m i xt u r e o ff l a m m a b l e o r c o m b u s t i b l e m a t e r i a l i n a i r ,m a y b e u s e d i n h a za r d o u s c l a s s i f i e d l o c a t i o n s .N EC A r t i c l e 504r e qu i r e s t h a t c o n d u c t o r s a n d c a b l e s o f i n t r i n s i c a l l y s a f e c i r c u i t s s h a l l b e p h y s i c a l l y s e p a r a t e d f r o m n o n -i n t r i n s i c a l l y s a f e c i r c u i t s .N o n i n c e n d i v e c i r c u i t s ,i n w h i c h a n y s p a r k o r t h e r m a l e f f e c t p r o d u c e d u n d e ri n t e n d e d o p e r a t i n g c o n d i t i o n s i s n o t c a p a b l e o f i g n i t i n g t h e g a s -a i r ,v a p o r -a i r ,o r d u s t -a i r m i xt u r e ,m a y b eu s e d f o r e qu i p m e n t i n Cl a s s 1 ,Di v i s i o n 2 a n d Cl a s s II,Di v i s i o n 2 h a za r d o u s c l a s s i f i e d l o c a t i o n s .

El e ctr ical Equip m e ntfor Cl ass IAp p l ications (gas/vap or )

Cl a s s I,Di v i s i o n 1 e qu i p m e n t i s u s e d w i t h t h e a s s u m p t i o n t h a t h a za r d o u s g a s e s o r v a p o r s w i l l b e p r e s e n t a n de v e n t u a l l y s e e p i n t o t h e e n c l o s u r e .Th e r e f o r e ,c o n s t r u c t i o n o f s u c h e qu i p m e n t m u s t b e s t r o n g e n o u g h t oc o n t a i n a n e xp l o s i o n w i t h i n ,a n d b e e xp l o s i o n p r o o f .It m u s t f u n c t i o n a t a t e m p e r a t u r e b e l o w t h e i g n i t i o nt e m p e r a t u r e o f t h e s u r r o u n d i n g a t m o s p h e r e p r o v i d i n g a w a y f o r t h e b u r n i n g g a s e s t o e s c a p e f r o m t h ed e v i c e a s t h e g a s e s e xp a n d d u r i n g a n i n t e r n a l e xp l o s i o n ,o n l y a f t e r t h e y h a v e b e e n c o o l e d o f f a n d t h e i rf l a m e s qu e n c h e d .Th e e s c a p e p a t h s c o u l d b e g r o u n d s u r f a c e o r t h r e a d e d f l a m e p a t h .Fu r t h e r ,i t i s i m p e r a t i v et o e n s u r e t h a t a l l f l a m e p a t h s a r e p r o t e c t e d d u r i n g h a n d l i n g ,s h i p p i n g ,s t o r a g e ,i n s t a l l a t i o n a n d m a i n t e n a n c eo f e xp l o s i o n p r o o f m a t e r i a l a n d e qu i p m e n t .

El e ctr ical Equip m e ntfor Cl ass IIAp p l ications

Cl a s s II,Di v i s i o n 1 e qu i p m e n t i s d e s i g n e d a n d c o n s t r u c t e d s u c h t h a t i t m u s t s e a l o u t t h e c o m b u s t i b l e d u s t ,o p e r a t e b e l o w t h e i g n i t i o n t e m p e r a t u r e o f h a za r d o u s s u b s t a n c e s ,a n d a l l o w f o r a d u s t b l a n ke t .Th e r e f o r e ,t h e r e i s n o n e e d f o r h e a v y e xp l o s i o n -p r o o f c o n s t r u c t i o n ,o r f l a m e p a t h s r e qu i r e d f o r e qu i p m e n t i n Cl a s s I,Di v i s i o n 1 l o c a t i o n s .Cl a s s IIe qu i p m e n t i s c a l l e d d u s t -i g n i t i o n p r o o f .

El e ctr ical Equip m e ntfor Cl ass IIIAp p l ications



There is very little difference in the design between Class II and Class III equipment. Class III equipment mustminimize entrance of fibers and flyings, prevent escape of sparks, burning material or hot metal particlesresulting from failure of equipment, and operate at a temperature that will prevent the ignition of fibersaccumulated on the equipment. Class III equipment is generally not used in coal-fired power plants.

There are many devices, fixtures and enclosures that are suitable for Class I, II and III locations. Additionally,a Class I device would have to prevent dust entering the enclosure to be suitable for Class III application.

Low Ambient Conditions Considerations

Explosion proof or dust ignition proofequipment is generally suitable for use in an ambient temperaturerange of -25C (-13F) to +40C (+104F). Such equipment may not be suitable for use at temperatures lowerthan -25C (-13F), unless they are identified for lower temperature service.

Equipment that is approved for Class I and Class II or Zone 0 and Zone 1 should be marked with themaximum safe operating temperature. Equipment must be selected so that its maximum surfacetemperature will be less than the ignition temperature of the coal dust.

Installation Considerations

Proper installation of electrical equipment in a hazardous location requires use of seals. Special fittings arerequired to keep hot gases from travelling through the conduit system igniting other areas if an internalexplosion occurs in a Class I device.

Coal handling buildings that have dust explosion hazards, structural members should be carefully reviewedfor explosion pressures. Also, proper pressure relieving vents and wall panels should be selected, locatedand provided.

Qualification of Electrical Equipment in Hazardous (Classified) Locations

There are a number of ways of protecting electrical equipment so that it does not cause an explosion whenused in a surrounding flammable atmosphere, or ignite a layer of dust on the equipment. The two mostcommon ways are explosion-proof equipment and dust-ignition proof equipment

Electric motors should be totally enclosed fan-cooled, Class II, Division 2/Zone 1 type or totally enclosed pipe-ventilated type, meeting temperature rise limitations.

In Coal handling areas, where accumulation of coal dust and its suspension in air are sources of potentialhazards, medium voltage switchgear and low voltage switchgear enclosures should be fully gasketed and beprovided with filtered and screened air.



Dust ignitionproof equipment should be enclosed in a manner that will exclude dust and constructed sothat arcs, sparks, or heat generated inside of the enclosure will not ignite exterior accumulation oratmospheric suspension of a specified dust on or in the vicinity of the enclosure.

Electrical devices including switches, solenoids, panels, should be used with NEMA 9 (dust ignitionproof)enclosures with watertight seals (O-rings), which together with the enclosed equipment in each case, shall beas complete assembly for Class II, Division 1, Group F locations.

NEC Article 500.8 (A) states that the suitability of identified equipment shall be determined by the equipmentlisting or labeling (for example, UL), evidence of equipment evaluation from a qualified testing laboratory orinspection agency, or evidence acceptable to the authority having jurisdiction.

Applicable Industry Codes and Standards

In the United States, codes and standards have been developed that are applicable to hazardous classifiedareas as shown in Table 1.

Table 1. Codes and standard applicable to hazardous classified areas. Source: Burns and Roe Enterprises.

(http://cdn.powermag.com/wp-content/uploads/2013/10/CP_090113_ElecClass_Table1.jpg)



Design Basis Document (DBD) and Drawings

A design basis document is an engineering document that defines the basis of engineering and design, andthe selection of electrical equipment that must meet requirements for each classified area. The design basisdocument also provides the necessary guidance to inter-discipline team of engineers in the design of theirsystems and design drawings for classified areas.

The design basis document and design drawings should include:

A listing of all combustible and flammable materials used in the facility, along with their pertinentproperties, such as ignition temperature, flash point, density and how and where handled.

Brief description of the process, operation, maintenance and cleaning procedures, and assumptions. A list, including dates and/or editions of all the codes, standards, references, practices, and other data

used to prepare area classification. A listing of each room or area and its determined area classification, along with the rationale for

making such determination. A complete set of electrical area classification drawings, indicating process flow, process equipment, the

normal leakage sources, and the boundaries of area classifications.

Practical Guidelines

Table 2 provides general practical guidelines for classification of electrical areas where combustible and/orflammable materials are located and processes are performed in a coal-fired power plant. The following is alist of NEC rules that are intended to convey an awareness of the complexity of electrical design in hazardousareas:

Explosion proof and dust-ignition proof equipment is required in Division 1 (Zone 0) areas and forcertain types of equipment in Division 2 (Zone 1) areas.

Approved equipment by UL or other appropriate agency should be preferred. Electrical equipment should preferably be located outside hazardous area. General-purpose equipment is permitted for certain applications in Division 2 (Zone 2) areas. Purging and pressurization of enclosures are permitted to prevent entrance of flammable and

combustible materials, provided the specific rules of NFPA 469 are followed. Instrument enclosures may be general-purpose type if they are part of intrinsically safe systems, and if

they are installed per NEC article 504. Rigid metal conduit is the allowable wiring method in Division 1 areas. Cable tray and cable, under certain conditions, are permitted in Division 2 areas. Electrical equipment must have temperature ratings or operating surface temperatures below the AIT

of the hazardous substance present.

Table 2. Guidelines for classification of electrical areas. Source: Burns and Roe Enterprises.



(http://cdn.powermag.com/wp-content/uploads/2013/10/CP_090113_ElecClass_Table2.jpg)



Sound engineering judgment should be applied and additional areas, not mentioned in Table 2, should beidentified. Equipment manufacturers recommendations for area classification of specific equipment shouldbe followed, if they are more stringent than the guidelines provided in Table 2.

When there is more than one leakage source in an area, such as in a manifold having several instruments,valves and flanges, or if there are several pieces of equipment with potential leak sources, the area should beboxed out as an overall 3 dimensional shape covering limits of extreme leakage points.

In addition to the recommended distances from sources, considerations should be given to use easilyrecognizable boundary limits when defining the horizontal and vertical extent of classified locations.Examples of recognizable boundaries are column lines, walls, ceilings, coordinates, equipment outlines,roads, dikes, etc. Areas identified by recognizable boundaries are helpful for plant installation, operation andmaintenance personnel.

To avoid undue expense, precaution should be taken to verify that boxing an overall area or extending aclassified area to recognizable boundaries does not include electrical equipment that would otherwise not beincluded in the hazardous area.

Project Documents

Project documents prepared and used in establishing hazardous areas include process flow diagrams of thesystems containing the hazardous materials, piping layouts, equipment general arrangements, and vendorsupplied equipment drawings

Two examples may be illustrative. The first shows a typical example of a pulverized coal-fired power plantcoal handling system process flow diagram (http://www.powermag.com/?attachment_id=43580), and thesecond shows boiler general arrangement with classified hazardous locations (http://www.powermag.com/?attachment_id=43582).

The design basis document and the associated design drawings that show the extent of horizontal andvertical boundaries of each classified area should be discussed with the owner, the construction contractorand the plant operators for their understanding of the affected areas. It also helps the plant operationpersonnel to take protective measures that ensure safety of operation and maintenance of plant and betterunderstanding to inspectors and insurance personnel.

Ram K. Saini, P.E. is assistant chief electrical engineer with Burns and Roe Enterprises.

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Electrical Area Classification in Coal-fired Power Plants

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Transcript of Electrical Area Classification in Coal-fired Power Plants